Adjusting method for recording condition and optical disc device

ABSTRACT

A method for evaluating reproduced signal wherein when Euclidean distance is calculated by judging the coincidence between a binary bit array and a predetermined evaluation bit array in the evaluation of the quality of reproduced signal, in a large capacity optical disc system with constraint length equal to or greater than 5, the assumption is made that the continuous 2T count included in a predetermined evaluation bit array is denoted by i and that each evaluation bit array is composed of a main bit array having a bit length of (5+2i) and auxiliary bit arrays added before and after the main bit array; judgment on whether binary bit arrays include the predetermined evaluation bit array, is concentrated on the coincidence judgment of the main bit arrays; and at the same time, the Euclidean distance between the reproduced signal and the target signal corresponding to the evaluation bit array is calculated with respect to each main bit array and the results of such calculations are separated and counted.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese applicationJP2009-097590 filed on Apr. 14, 2009, the content of which is herebyincorporated by reference into this application.

FIELD OF THE INVENTION

This invention relates to a method for adjusting recording condition inrecording information in an optical disc medium on which recorded marksare formed that have a physical property different from the physicalproperty of that part of the optical disc medium which is other than therecorded marks, and also relates to an optical disc device using theadjusting method.

BACKGROUND OF THE INVENTION

Optical disc media now widely available include CD-R/RW, DVD-RAM,DVD±R/Rw, BD, etc., and some of them have two data layers. Optical discdevices adapted for record and playback with those optical disc mediamentioned above, i.e. so-called DVD super multi-drives, are now widelyin use. In the near future, high-performance disc drives adapted to blueray discs (hereafter referred to simply as BDs) will come to be widelyused. Further, there's need for optical discs having still highercapacity.

The increase in the drive speed of optical disc drive and in the densityof information recorded in the optical disc has necessitated theintroduction of a technique in which the binarization of reproducedsignal is performed by Partial Response Maximum Likelihood (PRML)reproduction procedure. As one of the PRML procedures is known theadaptive PRML procedure (or system) or compensatory PRML procedure (orsystem) which can adaptively change the target signal level inaccordance with the reproduced signal. The non-patent document 1,“Journal C of Institute of Electronics, Information and CommunicationEngineers, Vol. J90-C, p. 519 (2007)” discloses the fact that a deviceadapted to BDs can achieve high-density recording equivalent to 35 GB byusing such a PRML procedure wherein the asymmetry of reproduced signaland the thermal interference at the time of recording can becompensated. It is pointed out in the document that reproductionperformance under the condition for high-density recording is higher forlarger constraint length (bit length representing ‘class’). In order toobtain the best result from binarization, an automatic equalizer thatmakes minimum the RMS error between reproduced signal and the targetsignal for the PRML index, is installed in an optical disc deviceprovided with such a PRML procedure. In general, such an automaticequalizer is installed as a Finite Impulse Response (FIR) filter havingits tap factors variable.

The increase in the recording density in an optical disc leads to thedecrease in the size of each recorded mark as compared with the size ofthe light spot, and therefore results in the reduction of the amplitudeof the obtained reproduced signal. The resolving power of the light spotis determined by the wave length λ and the aperture number NA of theobjective lens. Accordingly, if the length of the recorded mark havingthe shortest run length becomes equal to or less than λ/4NA, theamplitude of the signal corresponding to the repeated series of suchshortest recorded marks is reduced to zero. This phenomenon is known as“optical cutoff”, and may occur when λ/4NA≈119 nm in the case of BDs. Inthe case of BDs, an attempt to attain a recording capacity of more thanabout 31 GB with the track pitch kept constant, causes the amplitude ofthe signal corresponding to the repeated series of recorded marks havingthe shortest run length of 2T to be reduced to zero. It is thereforeindispensable to use a PRML procedure in order to acquire a satisfactoryreproduction performance under such a high-density condition.

When desired information is recorded in a recordable optical disc, thestate of crystallization, for example, in the recording film is changedby using pulsed laser light whose intensity is modulated (hereafterreferred to as ‘recording pulse’). Materials for such recording filmsinclude phase-variable substances, organic pigments, certain kinds ofalloys and oxides, all of which are well known and widely used. In themark edge coding method for use with CDs, DVDs and BDs, code informationis determined depending on the positions of leading and trailing edges.Regarding the recording pulses, the positions and widths of the firstpulse that mainly determines the condition for forming the leading edgeof a recorded mark and the last pulse that mainly determines thecondition for forming the trailing edge of the recorded mark, areimportant to maintain the quality of recorded information in a goodcondition. Therefore, it is customary with recordable optical discs touse “adaptive recording pulses” which can adaptively change thepositions or widths of the first and last pulses in accordance with thelength of each recorded mark and the lengths of the spaces thatimmediately precedes and follows the recorded mark.

Under such a high-density recording condition as described above, sincethe size of each formed recorded mark becomes very small, it isnecessary to choose the condition for radiating the recording pulses(hereafter referred to as “recording condition”) with a higher precisionthan conventional. On the other hand, in an optical disc device, theshape of the light spot varies depending on the wavelength at the lightsource, wave front aberration, focusing condition, the tilt of disc,etc. Further, since the ambient temperature and the aging effect changethe impedance and the quantum efficiency of the semiconductor laserdevice, the shapes of the recoiling pulses change accordingly. Thetechnique for invariably obtaining the best recording condition inresponse to the shapes of light spots and the shapes of the recordingpulses both of which fluctuate depending on environments and devices, isusually called “test writing”. Such a technique for adjusting therecording condition by using the test writing will become more and moreimportant with the requirement for further increasing recording density.

Adjusting techniques for recording condition are classified roughly intotwo categories: one method uses bit error rate or byte error rate asindex and the other utilizes statistical index such as jitter. Theformer pays attention to an event that occurs with a small probabilitywith respect to recorded data and the latter is concerned with theaverage quality of recorded data. Regarding write-once optical discs,for example, in the case where data are recorded in and reproduced fromplural locations in the disc with the recording condition varied, eventhe best recording condition for the former method may cause a large biterror or byte error if fingerprints overlie the recorded data.Therefore, the former method should not be selected in this case. Thebest recording condition should be such that the average quality of thedata recorded under such a recording condition is optimal. It cantherefore be said that the method using statistical index is preferablefor storage media such as optical discs, which are vulnerable tomaterial flaws, fingerprints or dust.

Methods corresponding to PRML procedure for statistically evaluating thequality of recorded data are disclosed in, for example, the non-patentdocument 2, “Jpn. J. Appl. Phys. Vol. 43, p. 4850 (2004)”; the patentdocument 1, JP-A-2003-141823”; the patent document 2,“JP-A-2005-346897”; the patent document 3, “JP-A-2005-196964”; thepatent document 4, “JP-A-2004-253114”; and the patent document 5,“JP-A-2003-151219”.

The patent document 1 discloses the technique wherein use is made of thecertainty Pa corresponding to the most likelihood state shift array andthe certainty Pb corresponding to the secondary likelihood state shiftarray so that the quality of reproduced signal is evaluated on the basisof the distribution of |Pa−Pb|. The non-patent document 2 discloses atechnique wherein the value obtained by subtracting the Euclideandistance between two target signals from the absolute value of thedifference between the Euclidean distance (corresponding to Pa) betweenthe target signal representing the binary bit array (corresponding tothe most likelihood state shift array) derived from the reproducedsignal and the reproduced signal, and the Euclidean distance(corresponding to Pb) between the target signal representing the binarybit array (corresponding to the secondary likelihood state shift array)derived through a single-bit shift of the interested edge and thereproduced signal, is defined as MLSE (Maximum Likelihood SequenceError), and the recording condition is adjusted in such a manner thatthe average value of the distribution of MLSEs is reduced to zero forevery recorded pattern.

The patent document 2 discloses a technique wherein edge shift isspecifically noted: a virtual pattern having a run length of 1T is usedas an error pattern for showing that the edge of reproduced signalshifts to the right or left; the amount of edge shift is obtained bycalculating the difference between sequence errors having plus or minussign depending on the direction in which the edge shift occurred; andthe recording condition is so adjusted as to cause the amount of edgeshift to approach zero. In this case, the evaluating index is called“V-SEAT (Virtual state based Sequence Error for Adaptive Target) index”.

The patent documents 3 and 4 disclose a technique wherein the differencebetween the Euclidean distance between reproduced signal and correctpattern and the Euclidean distance between reproduced signal and errorpattern, is calculated by using a table containing the combinations ofcorrect patterns and error patterns corresponding the correct patterns;and the Simulated bit Error Rate (SbER) is obtained from the average andstandard deviation of the Euclidean distance differences.

The patent document 5 discloses a technique wherein, on the basis of thedifference between the Euclidean distance between reproduced signal andcorrect pattern and the Euclidean distance between reproduced signal anderror pattern, the error probabilities corresponding respectively to thecase where the interested edge has shifted to the left and to the casewhere it has shifted to the right, are obtained; and the recordingcondition is so adjusted as to make the probabilities corresponding tothe two cases equal to each other. Accordingly, use is made of apreselected reproduced signal, a first pattern whose wave patterncorresponds to that of the preselected reproduced signal, and anarbitrary pattern (a second or a third pattern) whose wave patterncorresponds to that of the preselected reproduced signal but which isdifferent from the first pattern. First, the distance difference D=Ee−Eobetween the distance Eo between the reproduced signal and the firstpattern and the distance Ee between the reproduced signal and thearbitrary pattern, is obtained. Secondly, the distribution of thedistance differences Ds with respect to plural samples of reproducedsignals is obtained. Thirdly, the quality evaluation parameter (M/σ) isdetermined on the basis of the ratio of the average M of the obtaineddistance differences Ds to the standard deviation σ of the obtaineddistribution of the distance differences Ds. And finally, the quality ofreproduced signal is assessed from the evaluation index value (Mgn)represented by the quality evaluation parameters.

Prior Art Documents Patent Documents

-   Patent Document 1 JP-A-2003-141823-   Patent Document 2 JP-A-2005-346897-   Patent Document 3 JP-A-2005-196964-   Patent Document 4 JP-A-2004-253114-   Patent Document 5 JP-A-2003-151219

Non-Patent Documents

-   Non-patent Document 1 Journal C of Institute of Electronics,    Information and Communication Engineers, Vol. J90-C, P. 519 (2007)-   Non-patent Document 2 Jpn. J. Appl. Phys. Vol. 43, p. 4850 (2004)

SUMMARY OF THE INVENTION

The most likelihood state transition array and the secondary likelihoodstate transition array described in the patent document 1 are the sameas the correct and error patterns described in the patent document 3, inthat they are both the target bit arrays for measuring distances toreproduced signals. The patent documents 2 and 5 discloses three targetbit arrays, which are all the same in meaning. These target bit arrayswill hereafter be called an “evaluation bit array” collectively.Moreover, this invention basically aims to provide BD systems having ahigh capacity of more than 30 GB, and the following description will bemade on the assumption that the shortest run length for modulation codeis 2T.

As described in the non-patent document 1, a PRML system having aconstraint length of 5 or greater is preferable to achieve high-densityrecording. As described above, when recording density along the track(or linear recording density) is increased, the standard opticalrequirement for BD (wave length: 405 nm; aperture number of objectivelens: 0.85) will cause the amplitude of the signal repeated every 2Tperiod to be reduced to zero. In such a case, it is well known that thePRML system with PR(1,2,2,2,1) characteristic in which the amplitudes ofcondinuous 2T signals are reduced to zero, is suitable. The patentdocuments 3 and 4 disclose the method, adapted for PR (1,2,2,2,1)characteristic, calculating SbER (Simulated bit Error Rate) which isused for evaluating the quality of reproduced signal. This SbER methodutilizes the binarized bit arrays (correct patterns) and the secondarylikelihood evaluation bit arrays (error patterns) such as a bit arrayhaving Hamming distance of 1 from the correct pattern (an edge shift), abit array having Hamming distance of 2 from the correct pattern (shiftof 2T data) and a bit array having Hamming distance of 3 from thecorrect pattern (shift of 2T-2T data); regards the distribution of suchpatterns as Gaussian distribution; and estimates the bit error rate fromthe average and the standard deviation by using the error function.

Description will be made below of the performance required for thetechnique for adjusting the high-precision recording condition neededfor realizing an optical disc system having a recording capacity of 30GB or greater on the basis of the BD standard. Such a techniquerequires, with respect to at least the quality of the data recordedaccording to the result of adjustment, (1) that SbER and bit error rateshould be sufficiently small and (2) that the data recorded on a discwith a disc drive should give sufficiently small SbER and bit error ratewhen they are reproduced from the same disc with another disc drive. Therequired performance (1) given above seems to be a matter of course, butthe required performance (2) is specifically sought for since the discdrive must reproduce the data recorded with another disc drive.Therefore, it cannot be said that the adjusting method for recordingcondition which cannot meet the above described required performances(1) and (2) is suitable for the high-density optical disc system.

From the viewpoint of the two required performances described above,description will be made below of the problems associated with theconventional techniques and their combinations.

First, explained are various events caused when high-density recordingand reproducing having recording capacity of 30 GB or greater per discsurface is performed by way of experiments and simulations in whichlinear recording density is increased.

FIG. 2 graphically shows an experimental result that illustrates therelationship between bit error counts and recording power measured byusing a three layer wright-once optical disc which was produced fortesting purposes. The thin layers serving as recording medium on thetest disc were made of Ge-based chemical compound. The gaps between thetop and middle layers and between the middle and bottom layers were 14μm and 18 μm, respectively. The thickness of the transparent coveringlayer was 100 μm, measured from its top surface to its bottom surfacecontacting the bottom layer. The track pitch was 320 nm. Therecording/reproducing condition was such that data transfer speed isdouble that for BD and that the width (1T) of the detecting window wasset equal to about 56 nm. Accordingly, the desired recording density wasdesigned to be 33 GB. Laser beams of ordinary multi-pulse type modulatedfor three power levels, i.e. peak power, assist power and bottom power,were used as the sources of recording pulses. The processing system forreproduced signal comprised an 8-bit A/D converter, an automaticequalizer having 21 taps, and a Vitervi decoder having PR(1,2,2,2,1)characteristic. The minimum value for BER was less than 10⁻⁵ for each ofthe three layers. The peak power values for layers L0, L1 and L2, whichcaused BER to become minimal, was 13.5 mW, 15.5 mW and 11.5 mW,respectively. FIG. 2 graphically shows the relationship betweenrecording power and bit error counts while the recording power waschanged with the ratio among the three power levels maintained constant.This graph reflects the case where an edge shifts and also 1˜4consecutive 2T collectively shift (slip). It is seen from FIG. 2 thatthe bit error counts with respect to not only the edge shift but alsothe collective shifts are likewise larger than anticipated, in responseto the deviation of the recording power. This result is due to the factthat the amplitude of the 2T-2T signal is 0, and that Euclidean distanceof “12” for the collective shift of consecutive 2T is smaller thanEuclidean distance of “14” for the edge shift in the case ofPR(1,2,2,2,1) characteristic.

FIG. 3 graphically shows, as a result of simulation, the relationshipbetween SNR and SbER. In this simulation, the impulse responseobtainable when recorded marks are reproduced was obtained by a lineardiffraction simulator, and the reproduced signal obtainable when therecording is ideally performed was calculated by convolving the recordedbit array and the impulse response. Noise was added as white noise, andSNR was determined as the ratio of half the peak value of the8T-repetitive signal to the standard deviation of the noise. The desiredbit error rate and SbER were obtained through processing with areproduced signal processing system having PR(1,2,2,2,1) characteristic.The patent document 3 discloses the evaluation pattern corresponding tothe case where up to two continuous 2T signals continue. In thissimulation, up to six continuous 2T signals were caused to continue.This is an extension (Hamming distances of 1˜7) of the disclosed case.There are 18 evaluation patterns per Hamming distance so that the totalnumber of the evaluation patterns is 252. It is apparent from FIG. 3that the values of SbER remain almost constant when the number ofcontinuous 2T signals is equal to or greater than 2 (Hamming distance of3). Although this result seems to be contradictory to the experimentalresult shown in FIG. 2, it is not the case. The definition in thecalculation of SbER permits the estimation of bit error rate with theexistence probability of evaluation pattern taken into consideration,and even an evaluation covering up to two continuous 2T signals canestimate the overall bit error rate.

FIG. 4 graphically shows an experimental result indicating therelationship between bit error rate and SbER. An experiment was done insuch a manner that recording along five tracks is continued so as toinclude the influence of crosstalk in the L0 layer and that variousrecording/reproducing stresses are imposed on the track in the center.The stresses include the radial tilt (R-tilt) of disc, the tangentialtilt (T-tilt) of disc, the aberration of focus (AF), the sphericalaberration (SA) due to improper adjustment of the optical head beamexpander, and the change in the recording power (Pw). Regarding theradial tilt, the result with respect to the L2 layer is also shown. Itis apparent from FIG. 4 that there is a good correlation between biterror rate and SbER. The reason why there is a large scatter of datapoints around the bit error rate of 10⁻⁵ is ascribed mainly to the flawsin the recording material used in this experiment.

As is apparent from the result of the experiment and simulation, it isnecessary that under the requirement for the high-density recording andreproduction capable of achieving a recording capacity of 33 GB per discsurface, the evaluation of bit errors covering not only an edge shift(Hamming distance of 1) but also up to at least two continuous 2Tsignals (Hamming distance of 3) should be performed. The method ofevaluating the quality of reproduced signals, which concentrates only onthe edge shift, cannot be said to develop a sufficient correlationbetween bit error rate and SbER.

The distribution of Euclidean distance differences accompanyinghigh-density recording will now be described. The term “Euclideandistance difference” used in this specification signifies the valueresulting from subtracting the Euclidean distance between reproducedsignal and correct target signal from the Euclidean distance betweenreproduced signal and error target single. This Euclidean distancedifference is defined as |Pa−Pb| in the patent document 1 and as D inthe patent documents 3 and 4. Here, in order to consider an idealrecording state, such a simulation as described above was used. SNR wasset to 24 dB, and the distribution of the Euclidean distance differencescovering up to two continuous 2T signals was calculated while recordingdensity was changed within a range of 25 to 36 GB per disc surface(T=74.5 nm˜51.7 nm). The configuration of the reproduced signalprocessing system is as described above. FIG. 5 shows the result of suchcalculations. The obtained distributions are sometimes called the “SAMdistribution”. As described above, with PR(1,2,2,2,1) characteristic,the ideal Euclidean distance (=14) for an edge shift is different fromthe Euclidean distance (=12) for the shift of a 2T signal and theEuclidean distance (=12) for the shift of two continuous 2T signals. Andin order to denote them collectively, each Euclidean distance differencewas normalized by dividing it with the ideal Euclidean distance. In FIG.5, the statistical probability corresponding to the case where thedistance difference becomes zero (at the leftmost end) or negative,gives the bit error rate. As seen in FIG. 5, it is apparent that thedistribution spreads wider with the improvement in recording densitythough the SNR is kept constant. This tendency indicates the increase inerror rate with the increase in recording density, and therefore isquite reasonable. The average value of the distribution (nearly equal tothe peak value) for an edge shift remains the same at the position ofthe horizontal axis of near 1 (=ideal Euclidean distance) even when therecording capacity increases. However, the average value of thedistribution, i.e. the peak value, for the shift of a single 2T signalor two continuous 2T signals moves toward zere when the number of shiftsand the recording capacity increase. The cause of this phenomenon isconsidered to depend on the processing capacity of the automaticequalizer used. As described above, the automatic equalizer works insuch a manner that the RMS error between reproduced signal and correcttarget signal is minimized. Since the sampling interval is 1T that is afinite value, the calculation of discreet frequency characteristics canonly be performed up to half the sampling frequency according to theSampling Theorem. In this way, since the filter characteristicobtainable with an automatic equalizer is restricted as described above,the amplitudes of the higher harmonic components of the reproducedsignal become large for a recorded pattern in the region where a longcontinuation of 2T marks (spaces) occurs. As a result, it is consideredthat the upper limit of the processing capacity of the automaticequalizer is approached so that the deviation from the ideal Euclideandistance becomes large. As described later, the phenomenon that the peakvalue (or average value) of the distribution of Euclidean distancedifferences shifts with the increase in recording density, toward thedirection in which the peak value tends to be smaller than the idealEuclidean distance, is an important event concerning the technique foradjusting recording condition. It is noted here that none of the abovequoted documents have a description of this phenomenon.

On the basis of the above described experiment and simulation, from theviewpoint of the two required performances mentioned above, problemsarising from each or a combination of the conventional techniquesdescribed above are summed up as follows.

(1) Method Disclosed in the Non-Patent Document 2

The non-patent document 2 discloses the technique wherein edge shiftsare adjusted in such a manner that the average value of the distributionof the Euclidean distance differences becomes equal to the idealEuclidean distance, on the basis of the technique disclosed in thepatent document 1. The “Expression (1)” cited in the non-patent document2 defines a specific edge shift MD as a quantity given by the followingexpression (1).

$\begin{matrix}{{MD} = {{{{\sum\limits_{1}^{4}\left( {X - P_{A}}\; \right)^{2}} - {\overset{4}{\sum\limits_{1}}\left( {X - P_{B}} \right)^{2}}}} - d_{m\; i\; n}}} & (1)\end{matrix}$

In the above expression, X denotes the level of the reproduced signal;P_(A) and P_(B) are the target signal levels corresponding to thebinarized (i.e. binary) bit array (most likelihood state transitionarray) and a bit array with a one-bit edge-shift (secondary likelihoodstate transition array), respectively; and d_(min) is the Euclideandistance corresponding to the edge shift. By way of supplement,according to the result shown in FIG. 5, this method is one thatcorresponds to adjusting the recording condition in such a manner thatthe distribution of edge shifts takes the peak value at the idealEuclidean distance (=1). On the other hand, FIG. 3 illustrates the factthat under the condition for high-density recording the correlation withSbER (or bit error rate) is not sufficient if edge shifts alone aretaken into consideration. Consequently, it is understood from theviewpoint of the required performance (1) mentioned above that thismethod, which takes edge shifts alone into consideration, is notsatisfactory under the condition for high-density recording. Further,the “Table 2” in the non-patent document 2 reveals that there exists noadjusting index at the location where a 2T mark and a 2T space arejuxtaposed to each other, that is, the leading edge (Tsfp(2s, 2m)) of a2T mark following a 2T space and the trailing edge (Telp(2s, 2m)) of a2T mark followed by a 2T space. From this point of view along with theresult shown in FIG. 2, it cannot be said that this method issatisfactorily applied to a case of high-density recording condition inwhich the error for a 2T mark or space are considerable.

(2) Method Disclose in the Patent Document 2

This method disclosed in the patent document 2, too, takes edge shiftsalone into consideration to obtain the index for the adjustment ofrecording. However, if a virtual 1T mark or space is introduced, theadjustment of recording becomes possible also for the location where twocontinuous 2T patterns continue. Notwithstanding this, the method takesedge shifts alone into consideration and therefore cannot be said todevelop a sufficient correlation with SbER (or bit error rate).Accordingly, this method does not prove to be satisfactory, either, fromthe viewpoint of the required performance (1) mentioned above.

(3) Method Disclose in the Patent Document 5

According to the method disclosed in the patent document 5, even errorbit arrays are so selected as to satisfy the restriction on run lengthso that high correlation between index and SbER (or bit error rate) canbe developed for not only the case of an edge shift but also the casewhere two 2T patterns shift continuously. This method, therefore, can besaid to be an excellent method. According to this method, in order toadjust the recording condition involving a 2T mark, the Hamming distancebetween an error bit array to be evaluated and a correct bit array takesdifferent values according as the interested mark edge shifts leftwardor rightward, as shown in FIG. 3 of the patent document 5. For instance,let Tsfp(3s, 2m) be taken as an example in accordance with the notationemployed in the non-patent document 2. Then, those disclose bit arraysare as follows.

TABLE 1 left-shifted bit array 1 1 1 0 0 1 1 1 0 0 0 0 correct bit array1 1 1 0 0 0 1 1 0 0 0 0 right-shifted bit array 1 1 1 0 0 0 0 1 1 0 0 0

interested edge

In the case where the partial response characteristic is PR(1,2,2,2,1),the Hamming distance and the Euclidean distance, between the correct bitarray and the left-shifted bit array, are 1 and 14, respectively,whereas the Hamming distance and the Euclidean distance, between thecorrect bit array and the right-shifted bit array, are 2 and 12,respectively. As shown in FIG. 5, the difference in Hamming distancecauses the difference in the values for the average and the standarddeviation of distributions. In order to overcome this problem, thepatent document 5 introduces the concept of SbER, estimates their errorprobabilities by using the error function, and specifies the adjustmenttarget as the condition that their error probabilities are equal to eachother. According to this method, it is considered possible to determinesuch a recording condition as to minimize SbER (or bit error rate). Onthe other hand, the result of simulation as shown in FIG. 5 anddescribed above corresponds to the case where recorded marks are formedin the ideal condition (without any edge shift). As seen in FIG. 5, thechange in Hamming distance causes the change in the central value andthe standard deviation. Therefore, according to the method disclosed inthe patent document 5, the condition for forming recorded marks must beadjusted in such a manner that the error probabilities for threedistributions (probabilities for which Euclidean distance becomes 0) arerendered equal to one another. From the viewpoint of the requiredperformance (2) mentioned above, that is concerned with the warranty ofdisc compatibility, there is room for question of whether this method isan ideal one for adjusting recording condition for high-density opticaldiscs. For the purpose of quantitative evaluation on this point,analyses were made by using such a simulation as described above.

The extension of concept was introduced to define the amount of edgeshift to be detected by the method according to the patent document 5.According to the expression (13) in the patent document 5, the quantityEc equivalent to the edge shift is defined with the following expression(2).

Ec=(σ₃ *M ₂+σ₂ *M ₃)/(σ₂+σ₃)  (2)

In the above expression, M₂ and M₃, and σ₂ and σ₃ represent the averagesand the standard deviations of distributions of the Euclidean distancedifferences calculated when the interested edge is shifted by one bit tothe left and to the right, respectively. The result shown in FIG. 5 wasobtained by normalizing the two distributions with the ideal Euclideandistance, as described above. In like manner, if the ideal Euclideandistance is assumed to be equivalent to 1T, the edge shift Ec′ in thedirection of time axis can be calculated from the amount Ec equivalentto the edge shift by using M₂, M₃, σ₂ and σ₃ normalized with the idealEuclidean distance.

FIG. 6 shows the distributions obtained through simulation. It isevident from FIG. 6 that the same result as what is schematically shownin FIG. 6 of the patent document 5 has been obtained. FIG. 7 graphicallyshows the relationship between Ec′ and SNR with SNR changing. Asapparent from FIG. 7, the value of Ec′ increases rapidly with theincrease in SNR. With optical disc devices, the shape of the light spotand the SNR of the optoelectronic transducer change depending on thetypes of devices or in response to, for example, ambient temperatures.For a storage device such as a hard disc drive in which the disc mediumis unchangeably installed, it is the best method to adjust the recordingcondition in such a manner that SbER (or bit error rate) with respect tothe hard disc drive of interest is minimized. However, for a storagesystem such as an optical disc device in which the disc medium isinterchangeable, it is not satisfactory to minimize SbER (or bit errorrate) with respect only to that particular device. From the viewpoint ofthe required performance (2) mentioned above, this method still leavesroom for improvement in pursuing the optimal method for obtainingrecording condition for high-density recording.

Further, the reason why this method leaves room for improvement from theviewpoint of the required performance (1) will also be described. Thebit arrays used for evaluating Tsfp(3s, 2m) are as described above. Onthe other hand, the following bit arrays for evaluation are also usedfor calculating SbER, as described in the patent document 4.

TABLE 2 left-shifted bit array 1 1 1 0 0 1 1 1 0 0 1 1 1 correct bitarray 1 1 1 0 0 0 1 1 0 0 1 1 1 right-shifted bit array 1 1 1 0 0 0 0 11 0 0 1 1

interested edge

Table 2 given above corresponds to the case where the interested 2T markis immediately followed by a 2T space. With respect to the left-shiftedbit array, the Hamming distance and the Euclidean distance, from thecorrect bit array are 1 and 14, respectively, just as described above,whereas with respect to the right-shifted bit array, the Hammingdistance and the Euclidean distance, from the correct bit array are 3and 12, respectively. The Hamming distance in this case differs fromthat in the previous case. From the viewpoint of the requiredperformance (1), it is expected that the correlation between theevaluation index for recording adjustment and the index SbER (or biterror rate) for evaluating the quality of reproduced signals, issufficiently strong. Therefore, it is necessary that the indexrepresenting the evaluation bit array for recording adjustment isroughly the same as the index for evaluating the quality of reproducedsignals. The patent document 5 does not disclose any measure for solvingthe problem that with respect to the evaluation index using the targetsignal corresponding to the bit array in which the interested edge isshifted to the left or to the right, there arises, as in this instance,plural combinations of (Hamming distance 1 for left edge shift andHamming distance 2 for right edge shift) and (Hamming distance 1 forleft edge shift and Hamming distance 3 for right edge shift). From thispoint of view, too, it can be said that this method leaves room forimprovement.

(4) Method According to Combination of Conventional Techniques

The non-patent document 2 discloses a technique wherein, on the basis ofthe technique disclosed in the patent document 1, edge shifts are takeninto consideration and adjustment is made in such a manner that theaverage of distributions of Euclidean distance differences becomes equalto the ideal Euclidean distance. An analogy of a method is easy formaking such adjustment that the average of distributions becomes equalto the ideal Euclidean distance, by applying this technique disclosed inthe non-patent document 2 to the evaluation bit arrays shown in “FIG. 3of the patent document 5”. As shown in FIG. 5, however, the increase inrecording density causes the average of the respective distributions todeviate in the direction in which the average tends to be smaller ascompared with the ideal Euclidean distance. In like manner, the averageof the respective distributions change in response also to SNRs. FIG. 8shows the result of the experiment made to ascertain this phenomenon.This experimental result was obtained in reproducing the recordedsignals while changing the reproducing (or read) power for the layer L0of the above described test disc having three layers. In FIG. 8, thehorizontal axis is graduated in reproducing (or read) power with 1.2 mWequated to 100%. The amplitude of the reproduced signal is proportionalto the reproducing (or read) power, but the noise (amplifier noise)inherent to the photodetector is constant. In this experiment, the SNRof the reproduced signal is varied by changing the reproducing (or read)power. It is understood from FIG. 8 that the average of the respectivedistributions is smaller than the Euclidean distance (=1) and that theaverage decreases with the decrease in the reproducing (or read) power.It is apparent with this method, too, that the difference in SNRdepending on the states of drive devices affects the index used toadjust recording.

(5) Method for Minimizing SbER

As shown in FIG. 4, SbER develops a strong correlation with bit errorrate in the experiment regarding recording density of 33 GB per discsurface. Accordingly, a candidate method is to select the condition forobtaining the minimum SbER after having performed write/read operationsunder all the combinations of possible recording conditions withoutusing the evaluation index for write adjustment. However, it issubstantially impossible to search for the condition for obtaining theminimum SbER while randomly changing the recording conditions, in such acase as of an optical disc medium in which the size of the area (testwrite area) for write adjustment is limited. For it is impossible toobtain the information on the direction for making the edge of therecorded mark approach the ideal state. Except for the conventionaltechniques as described above which can quantitatively determine thedeviations from the target value independently in response to therespective parameters of recording pulses, no method can serve toperform test write applicable to optical disc devices. Further, even inthe case where performance improvement is sought while repeating thetest fabrication of discs, it is desirable to complete the adjustment ofrecording condition in a short period of time. In this sense, too, theinvention of a novel index and a novel adjusting method has long beenwaited for according to which the required performances (1) and (2)mentioned above are satisfied and the above mentioned quantitativedetermination of deviations can be adjusted independently in response tothe parameters for recording.

As described above, regarding the adjustment of recording conditioncorresponding to the high-density recording condition with recordingcapacity of more than 30 GB per disc surface in case of a BD system,there was a problem with conventional techniques that the performance ofadjustment is not sufficiently compatible with the guarantee ofinterchangeability of recording media. The objects of this invention areto provide a novel index and a novel method for the adjustment ofrecording that can solve this problem and to provide an optical discdevice using them.

According to this invention, since the achievement of a large capacityof more than 30 GB is aimed for, it is assumed in the followingdescription of this specification that the minimum run length of themodulation code is 2T. Further, as described above, since theexperimental results indicate that the SbER used in the case where up totwo 2T continue, coincides well with the bit error rate, it is assumedthat SbER is used as the index for evaluating the quality of reproducedsignal when the evaluation index for record adjustment according to thisinvention is discussed. The index for probabilistically evaluating thequality of reproduced signal on the basis of the Euclidean distancebetween the target signal and the reproduced signal, or the index fordirectly evaluating bit error rates can yield a good result if therecording condition is adjusted according to this invention.

The summary of the above described requirements is as follows.

[Requirement 1] Compatibility in Reproduction of Data Recorded on theBasis of Result of Adjustment

The evaluation index and the method of adjustment to be employed mustnot depend on the variation of SNR, but must concentrate on a fixedadjusting target.

[Requirement 2] Quality of Data Recorded on the Basis of Result ofAdjustment

In order to secure that SbER is sufficiently small, the evaluation bitarray covering at least up to two continuous 2T patterns must coincideexactly or substantially with the evaluation bit array for SbER.

[Requirement 3] Completion of Record Adjustment in a Short Period ofTime

The evaluation index and the method of adjustment, capable ofindependent evaluation, must be provided in accordance with theconditions of recording pulses or the respective parameters of adaptiverecording pulses.

According to the concept of this invention, evaluation is performed byseparating the component corresponding to the shift of the interestededge from the component depending on SNR in accordance with thedifference between the Euclidean distances from the reproduced signal tothe two target signals. In order to facilitate the understanding of thisinvention, the definition of the evaluation index that can solve theseproblems will first be given, and then the fact will be described thatthe evaluation index can solve the problems.

In the following description of the specification, W denotes areproduced signal, T a target signal in the form of a binary bit arrayobtained from the reproduced signal, L a target signal in the form of abinary bit array in which the interested bit is shifted by a single bitto the left and which satisfies the condition of restriction on runlength, and R a target signal in the form of a binary bit array in whichthe interested bit is shifted by a single bit to the right and whichsatisfies the condition of restriction on run length. The Euclideandistances between W and T, between W and R, etc. are denoted by ED(W,T), ED(W, R), etc. The evaluation value for the error caused when theinterested edge is shifted leftward is represented by xL, and theevaluation value for the error caused when the interested edge isshifted rightward is represented by xR. These evaluation values arecalled “equivalent edge shifts” and defined with the followingexpressions (D1) and (D2).

$\begin{matrix}{{xL} = {\frac{1}{2}\left( {1 - \frac{{{ED}\left( {L,W} \right)} - {{ED}\left( {T,W} \right)}}{{ED}\left( {T,L} \right)}} \right)}} & ({D1}) \\{{xR} = {\frac{1}{2}\left( {1 - \frac{{{ED}\left( {R,W} \right)} - {{ED}\left( {T,W} \right)}}{{ED}\left( {T,R} \right)}} \right)}} & \left( {D\; 2} \right)\end{matrix}$

The amount of edge shift of the interested edge is called “extended edgeshift D” and defined with the following expression (D3).

D=(xR−xL)/2  (D3)

The amount of compensation equivalent to the error probability for theinterested edge is called “SNR factor S” and defined with the followingexpression (D4).

S=(xR+xL)/2  (D4)

Regarding the interested edge and the group of edges recorded with thesame record condition, that is, with a recording pulse that creates arecorded mark having a mark length equal to the space length of a spaceammediately anterior (or posterior) to the mark, the amount of edgeshift used for record adjustment, which is interpreted as thestatistical average Δ of extended edge shifts Ds, is defined with thefollowing expression (D5).

$\begin{matrix}{\Delta = {\sum\limits_{n = 1}^{N}D_{n}}} & ({D5})\end{matrix}$

where N denotes the total number of edges subjected to measurement, andD_(n) the extended edge shift for the n-th edge.

Further, the amount of jitter denoted by σ, equivalent to the errorprobability for the interested edge, is defined with the followingexpression (D6).

$\begin{matrix}{\sigma = \sqrt{\frac{1}{N}\left( {{\sum\limits_{n = 1}^{N}D_{n}^{2}} + {\sum\limits_{n = 1}^{N}S_{n}^{2}}} \right)}} & ({D6})\end{matrix}$

where S_(n) is the SNR factor for the n-th edge.

The quantities, i.e. the “evaluation indices” used in this invention,defined above with the expressions (D1)˜(D6) are each called L-SEAT(run-length-Limited Sequence Error for Adaptive Target) index.Specifically, Δ defined with the expression (D5) is called L-SEAT shift,and σ defined with the expression (D6) is called L-SEAT jitter.According to the adjusting method for the recording condition employedin this invention, recording/reproducing is performed while changing theconditions for recording pulses, and such a particular pulse conditionfor recording is selected that both the absolute value of the L-SEATshift and the value of L-SEAT jitter, for the interested edge areminimized.

Now description is made to ascertain that the above describedRequirements 1˜3 are satisfied by the adjusting method for recordingcondition according to this invention. As described in the patentdocuments 1-5 mentioned above, when PRML procedure is used, the errormargin is expressed by the Euclidean distance. In order to facilitatethe descriptions to follow, the quantities dEDL and dEDR, which are thevalues obtained by normalizing the Euclidean distance differences forthe errors caused when the interested edge is shifted to the left and tothe right, respectively, with the ideal Euclidean distance are definedwith the following expressions (3) and (4).

$\begin{matrix}{{dEDL} = \frac{{{ED}\left( {L,W} \right)} - {{ED}\left( {T,W} \right)}}{{ED}\left( {T,L} \right)}} & (3) \\{{dEDR} = \frac{{{ED}\left( {R,W} \right)} - {{ED}\left( {T,W} \right)}}{{ED}\left( {T,R} \right)}} & (4)\end{matrix}$

[Requirement 1] Compatibility in Reproduction of Data Recorded on theBasis of Result of Adjustment

As described above, the evaluation index for record adjustment must beconstant independent of the change in SNR. The average value ofdistributions of respective Euclidean distance differences changes inresponse to the change in SNR. Since W, T, L and R denote the signallevels at plural time instants t (t=t₀+1, t₀+2, t₀+3, t₀+4, t₀+5), letthem represent the coordinate points in the multi-dimensional space. Forthe sake of simplicity, an example is taken of the error caused when arightward shift of edge occurred and having the resulted Hammingdistance of 1. With the (1,2,2,2,1) characteristic, it can follow that T(T₁, T₂, T₃, T₄, T₅), W (T₁+δ₁, T₂+δ₂, T₃+δ₃, T₄+δ₄, T₅+δ₅) and R (T₁+1,T₂+2, T₃+2, T₄+2, T₅+1). Further, when the five-dimensional coordinatesystem is introduced having its origin at T, the position vectors(=coordinate points) for W and R may be represented as W (δ₁, δ₂, δ₃,δ₄, δ₅) and R (1, 2, 2, 2, 1). FIG. 9A illustrates the relativepositions of the coordinate points T, W and R in the plane containingthese three points. In FIG. 9A, the x-axis extends in the directionalong the line segment TR, and normalization is introduced so as tolocate the point R at 1 on the x-axis. Further, it should be noted thatsince the y-axis is taken perpendicular to the x-axis, the direction ofthe y-axis does not remain fixed but changes depending on the value ofW. The Euclidean distances among W, T and R are such as given by thefollowing expression (5).

ED(T,W)+ED(R,W)≧ED(T,R)  (5)

Namely, the sum of the Euclidean distance between T and W and theEuclidean distance between R and W is not necessarily equal to theEuclidean distance between T and R.

FIG. 9B schematically illustrates the measurement of the edge shift of aphysically recorded mark. In FIG. 9B, if the distance measured from thetarget value (the origin) T to the edge of the recorded mark is given byx, then the distance from the edge of the recorded mark to the targetvalue R, which is equivalent to the position shifted by 1T to the rightfrom the origin, becomes equal to (1−x). And the sum of the distancesalways becomes equal to 1 (=1T: T equals the width of the detectionwindow). In general, the edge control by means of recording pulses isthe control of shift along the time axis, being based on the idea oflinear measurement for the edge shift of physically recorded marks.

Accordingly, in the definition of the Euclidean distance (equal to thesquare of the length of line segment), too, if the projection of thevector TW onto the x-axis is represented by xR, then the projection ofthe vector RW onto the x-axis becomes equal to (1−xR), the sum of theseprojected components of the vectors TW and RW being equal to 1. Theinner product of the vectors TW and TR can produce xR, which can becalculated by using the Euclidean distances among the coordinate pointsT, R and W, by the following expression (6).

$\begin{matrix}\begin{matrix}{{xR} = {\delta_{1} + {2\delta_{2}} + {2\delta_{3}} + {2\delta_{4}} + \delta_{5}}} \\{= {\frac{1}{2}\left( {1 - \frac{\begin{matrix}{\begin{Bmatrix}{\left( {1 - \delta_{1}} \right)^{2} + \left( {2 - \delta_{2}} \right)^{2} + \left( {2 - \delta_{3}} \right)^{2} +} \\{\left( {2 - \delta_{4}} \right)^{2} + \left( {1 - \delta_{5}} \right)^{2}}\end{Bmatrix} -} \\\left( {\delta_{1}^{2} + \delta_{2}^{2} + \delta_{3}^{2} + \delta_{4}^{2} + \delta_{5}^{2}} \right)\end{matrix}}{14}} \right)}} \\{= {\frac{1}{2}\left( {1 - \frac{{{ED}\left( {R,W} \right)} - {{ED}\left( {T,W} \right)}}{{ED}\left( {T,R} \right)}} \right)}} \\{= {\frac{1}{2}\left( {1 - {dEDR}} \right)}}\end{matrix} & (6)\end{matrix}$

This item obtained by the above expression (6) is what is meant by theequivalent edge shift xR defined with the expression (D2) given above.The calculation of equivalent edge shift in the case where the Hammingdistance is 2 or 3 can be likewise performed. The second term in theexpression (6) indicates the Euclidean distance difference normalizedwith the ideal Euclidean distance, as shown in FIG. 5. The quantity xRis not only the projection of W along the direction of the line segmentTR but also the quantity related to the error probability that is one ofthe PRML indices. As a result of natural extension of concept, theequivalent edge shift xL can be calculated by using the target value L,which is equivalent to the position shifted by 1T to the left from theorigin, through the expression (D1) given above.

On the other hand, since the coordinates of W change depending on thevalue of SNR, the value of the equivalent edge shift changes dependingon edges to be measured. However, as described above, since the linearaddition of equivalent edge shifts is possible in the direction alongthe line segment TR, it becomes possible to evaluate, independent ofSNR, the edge shift of a recorded mark by calculating the average valueof the equivalent edge shifts.

The method for coping with the subject that the average of Euclideandistance differences changes depending on SNR will be described below.As described above, the main factor of this phenomenon seems to beascribed to the fact that the frequency characteristic of the filterimplemented by an automatic equalizer is restricted by the SamplingTheorem. Accordingly, when a specific edge is considered, the variationsof the average caused as the specified edge shifts to the left and tothe right, respectively, are equal to each other. This can be surmisedfrom the fact that the variations of the average of distributions can beclassified in terms of the continuous 2T count, i.e. Hamming distances,as shown in FIG. 5. Let the averages of the normalized Euclideandistance differences dEDL and dEDR be denoted respectively by M_(L) andM_(R), the respective deviations thereof from the ideal Euclideandistance by dM, and the amount of shift to be measured by Δ₂. Then, thefollowing equations (7) and (8) hold.

M _(L)=1−Δ+dM  (7)

M _(R)=1+Δ+dM  (8)

On the other hand, according to the technique disclosed in the patentdocument 2, which uses V-SEAT, the normalized sequence errors arecalculated depending solely on the edge shifts (Hamming distance 1),plus or minus signs is given the normalized sequence errors with respectto the left or right edge shift, and the arithmetic average of thesigned sequence errors is calculated. For example, it is quite naturalthat the equivalent edge shift to the right is given a plus (+) sign andthe equivalent edge shift to the left is given a minus (−) sign. On thebasis of this convention of sign allocation, the equivalent edge shiftsare calculated with respect to the leftward and rightward shifts of theinterested edge. If the arithmetic average of the signed equivalent edgeshifts whose signs are given with respect to the edge shift to the leftor right, is used as an evaluation value, then the variation dM of theaverage of distributions of Euclidean distance differences depending onSNR can be offset.

(M _(R) −M _(L))/2=Δ₂  (9)

In like manner, it is understood that the extended edge shift D, whichis defined with the expression (D3) as a measured value for aninterested edge, is the evaluating value for edge shift free of theinfluence depending on SNR. The L-SEAT edge shift Δ defined with theexpression (D5) is statistically equivalent to the difference Δ₂ of therespective distribution averages, the Δ₂ being defined with the aboveexpression (9).

FIGS. 10A and 10B diagrammatically show the left and right equivalentedge shifts xL and xR. In these figures, the coordinates of L, R and Ware represented in the six-dimensional space of t (t=t₀, t₀+1, t₀+2,t₀+3, t₀+4, t₀+5) with T assumed as the origin, in consideration of onetime instant by which L and R are staggered from the origin. The edgeshift x of a physically recorded mark is given by x={(1−x)+(1+x)}/2where (1−x) is the distance from the anterior edge of the mark to thepoint R which is staggered by 1T to the right from the point T, and(1+x) is the distance to the anterior edge of the mark from the point Lwhich is staggered by 1T to the left from the point T. Expression (9)means this and measurement. On the other hand, regarding the Euclideandistances among W, T, L and R, since L and R are staggered in time, theline segment TR and the line segment TL are not in alignment with asingle geometrical straight line. The angle θ subtended by the two linesegments can be given by the inner product of two vectors. If they areerrors each of which corresponds to an edge shift (Hamming distance 1),then cos θ is given by the following expression (10).

$\begin{matrix}\begin{matrix}{{\cos \; \theta} = \frac{{{vector}\left( {T,L} \right)} \cdot {{vector}\left( {T,R} \right)}}{{{{vector}\left( {T,L} \right)}}{{{vector}\left( {T,L} \right)}}}} \\{= \frac{0 - 2 - 4 - 4 - 2 - 0}{\sqrt{14} \times \sqrt{14}}} \\{= {- \frac{12}{14}}}\end{matrix} & (10)\end{matrix}$

In the above expression, vector (T, L) and vector (T,R) represent theposition vectors of L and R target signal, respectively, and theoperator “·” indicates “inner product”. If T is the most likely targetsignal and if L and R are the secondary likely (having highest errorprobability) target signals, then it is reasonable from the viewpoint oferror rate in PRML procedure that the recording condition should be soadjusted as to reduce the extended edge shift D to zero. The fact thatthe two target signals are not on a single geometrical straight line canbe said to be the feature of edge shift measurement according to PRMLprocedure. When the continuous 2T count is 2 (Hamming distance 1, 2 and3), the relationship among L, R and cos θ is summarized in FIG. 11. Itis seen from FIG. 11 that if the Hamming distance of L is 1 and theHamming distance of R is 3, then cos θ>0, indicating the angle betweenvectors L and R being less than 90 degrees, but that if target signalshaving the highest error probability are selected as L and R, then theedge shift of the interested edge can be measured on the basis of theaverage Δ of the extended edge shifts D's or the difference Δ₂ of theaverages of L- and R-distributions.

FIGS. 12A˜12D graphically show the relationships between dEDL and dEDR,obtained as a result of simulations. The conditions for the simulationswere such that the recording density was of 33 GB per disc surface andmarks having predetermined lengths were ideally recorded. In thesesimulations, SNR was equated to 20 dB. FIGS. 12A˜12D correspondrespectively to such front edges of marks as (a) Tspf(2s, 2m), (b)Tspf(2s, 3m), (c) Tspf(3s, 2m) and (d) Tspf(3s, 3m), in each of whichdata were collected from 1000 edges. In these simulations, the targetsignals having Hamming distances of (a) (2,2), (b) (2, 1) and (3, 1),(c) (1, 2) and (1, 3) and (d) (1, 1) were used as the L and R targetsignals. The dashed straight lines in these figures indicate therelationship dEDL+dEDR=2, i.e. relationship equivalent to theconservation of values measured for the physically recorded mark shownin FIGS. 10A and 10B. It is seen from these figures that the plotteddata points are scattered roughly along the dashed straight lines,indicating that they develop correlations between dEDL and dEDR, andthat the fluctuations with respect to the left and right edge shifts ofreproduced signals due to noise are roughly symmetric. To be precise, asseen from FIGS. 12B and 12C, the data distributions indicate gradients alittle different from the gradients of the dashed straight lines whenthe Hamming distances of L and R are not equal to each other. This isbecause the probabilities of error occurrence for the left and rightshifts are different from each other according to PRML procedure, thatis, because the measurement of physically recorded marks is differentfrom the measurement on the basis of error margins according to PRMLprocedure. The evaluation of edge shifts using V-SEAT disclosed in thepatent document 2 adopts only the target signals having Hamming distance1 and therefore can only produce the measurement results in which datadistribution gradients are parallel to the dashed straight lines even inthe cases of Tsfp(2s, 3m) and Tsfp(3s, 2m) of FIGS. 12B and 12C,respectively. The first advantage according to this invention is to haveovercome this point.

Each of FIGS. 13A˜13D graphically shows the relationship between theaverage of dEDL and dERR and the extended edge shift D, obtained as aresult of simulations. The conditions for the simulations were the sameas in the simulations shown in FIGS. 12A˜12D. These pictures alsocorrespond to (a) Tspf(2s, 2m), (b) Tspf(2s, 3m), (c) Tspf(3s, 2m) and(d) Tspf(3s, 3m), for each of which data were collected from 1000 edges.In these figures, the distributions of averages of dEDL and dEDR (1)spread widely and differently for the different edge patterns and (2)are all shifted toward values smaller than the ideal Euclidean distancedifference (=1). This tendency reflects the results shown in FIG. 5. Incontrast to this, the distributions of extended edge shifts D's do notdepend on edge patterns, and (1) the spreads of distributions are almostuniform and (2) the center of the spread of each distribution is locatednearly at zero. In these figures, these differences are representeddiagramatically by different spread shapes. These two advantages (oreffects) obtained with the introduction of extended edge shifts D's aredue respectively to (1) the calculation of the shift of reproducedsignal as the inner product of equivalent edge shift and vactor TR or TLand the ensuing linearization, and (2) the averaging of the left andright equivalent edge shifts with minus and plus signs attached thereto,respectively.

FIG. 14 graphically shows the summary of the advantages obtainedaccording to this invention. FIG. 14 shows the relationship between Ec′(adopted in the method disclosed in the patent document 5) and SNR, asshown in FIG. 7, superposed with the relationship between averages Δ(defined with the expression (D5)) of extended edge shifts and SNR.Since the optical disc device handles a portable media, it is requiredto have a function capable of executing a recording to a plurality ofvarious media in the same recording condition. However, SNR of actuallyrecorded information varies according to operational environment of theoptical disc device and condition of machine. In a convention method, asshown in FIG. 14, the value of Ec′ varies largely depending on thevariation of SNR. This means that the difference between an idealrecording signal and an actually recorded recording signal (In FIG. 14,it is evaluated using edge shift amount as barometer) varies accordingto an operational condition (SNR) of the optical disc device. On theother hand, in the present invention, the Δ value is almost zero andconstant without depending on the variation of SNR. This means thatrecording was executed in the same condition though the operationalcondition of the optical disc device varied. As stated abobe, in thepresent simulation, random noise are added to a signal corresponding toa case that a recording mark of a predetermined length is ideallyrecorded. The measurement result that the evaluation value Δ for edgeshift is nearly equal to zero under such condition indicates highexcellence of the method of this invention from the viewpoint ofcompatibility in the reproduction of recorded data. This point is thesecond advantage according to this invention.

[Requirement 2] Quality of Data Recorded on the Basis of Result ofAdjustment

As a result of the adjustment of recording condition according to thisinvention, SbER must be sufficiently small. In order to realize thisrequisite, it is necessary that dEDL and dEDR are minimized through theadjustment of recording pulses and that the evaluation bit arrays for T,L and R are substantially equivalent to the evaluation bit array forSbER.

The former need will first be described. It has been already describedthat all of the target signals T, L and R are not on a geometricalstraight line since they have different Hamming distances and staggeredin time from one another. Accordingly, the absolute value of theequivalent edge shift for the edge shift to the left is different fromthat for the edge shift to the right. This is the feature of the edgeshift measurement according to this invention. Now, in evaluating Nedges, let dEDL and dEDR of the n-th edge be denoted by dEDL_(n) amddEDR_(n), and let their average be approximated by 1. The standarddeviations σ_(L) and σ_(R) can be represented by the followingexpressions (11) and (12), respectively.

$\begin{matrix}{\sigma_{L} = \sqrt{\frac{1}{N}{\sum\limits_{n = 1}^{N}\left( {{dEDL}_{n} - 1} \right)^{2}}}} & (11) \\{\sigma_{R} = \sqrt{\frac{1}{N}{\sum\limits_{n = 1}^{N}\left( {{dEDR}_{n} - 1} \right)^{2}}}} & (12)\end{matrix}$

The bit error rate can be evaluated by using the synthetic standarddeviation σ_(LR) of them represented by the following expression (13).

$\begin{matrix}\begin{matrix}{\sigma_{LR} = \sqrt{{\frac{1}{2N}{\sum\limits_{n = 1}^{N}\left( {{dEDL}_{n} - 1} \right)^{2}}} + \left( {{dEDR}_{n} - 1} \right)^{2}}} \\{= \sqrt{\begin{matrix}{{\frac{1}{4N}{\sum\limits_{n = 1}^{N}\left\{ {\left( {{dEDR}_{n} - 1} \right) - \left( {{dEDL}_{n} - 1} \right)} \right\}^{2}}} +} \\{\frac{1}{4N}{\sum\limits_{n = 1}^{N}\left\{ {\left( {{dEDR}_{n} - 1} \right) + \left( {{dEDL}_{n} - 1} \right)} \right\}^{2}}}\end{matrix}}} \\{= \sqrt{\begin{matrix}{{\frac{4}{N}{\sum\limits_{n = 1}^{N}\left( \frac{\frac{1 - {dEDR}_{n}}{2} - \frac{1 - {dEDL}_{n}}{2}}{2} \right)^{2}}} +} \\{\frac{4}{N}{\sum\limits_{n = 1}^{N}\left( \frac{\frac{1 - {dEDR}_{n}}{2} + \frac{1 - {dEDL}_{n}}{2}}{2} \right)^{2}}}\end{matrix}}} \\{= {2\sqrt{{\frac{1}{N}{\sum\limits_{n = 1}^{N}\left( \frac{{xR} - {xL}}{2} \right)^{2}}} + {\frac{1}{N}{\sum\limits_{n = 1}^{N}\left( \frac{{xR} + {xL}}{2} \right)^{2}}}}}} \\{= {2\sqrt{\frac{1}{N}\left( {{\sum\limits_{n = 1}^{N}D_{n}^{2}} + {\sum\limits_{n = 1}^{N}S_{n}^{2}}} \right)}}}\end{matrix} & (13)\end{matrix}$

The right-hand side of the above expression (13) becomes equal to doublethe value of the L-SEAT jitter given by the expression (D6). The factor2 appearing on the right-hand side is not essential, but results fromthe fact that the error margin is ±1/2T for L-SEAT as in the case ofjitter measurement by the conventional time interval analyzer, whereasthe error margin is 1 (ideal Euclidean distance=1) in the distributionsof dEDL and dEDR. If the distributions of dEDL and dEDR are regarded asof Gaussian, the error rates for these distributions obtained by usingthe error function become equal to each other. It is apparent that theL-SEAT jitter represents the synthetic standard deviation obtained bysuperposing the distributions of the Euclidean distance differencesshown in FIG. 5 in such a manner that the average of the distributionsoccurs at the ideal Euclidean distance (=1). It can therefore be saidthat the L-SEAT jitter is the evaluation index that exhibits a strongcorrelation with SbER or bit error rate. To be further precise, as seenfrom the expression (D4), the SNR factor by definition has as itsaverage a value equal to the quantity by which the distribution ofEuclidean distance differences is deviated from the ideal value (=1)depending on SNR and recording density. Therefore, the contribution ofSNR to the L-SEAT jitter defined with the expression (D6) involves thedeviation of the average of the distributions of Euclidean distancedifferences. As described above, the L-SEAT jitter according to thisinvention can be evaluated by separating the component corresponding tothe shift of the interested edge (the extended edge shift) from thecomponent depending on SNR (the SNR factor). In this way, two functionscan be simultaneously provided: one is the shift adjustment excellent inreproduction compatibility independent of SNRs of different drivedevices, and the other is the warranty of rendering SbER and bit errorrate to the minimum. Namely, in comparison with those conventionalsignal evaluation indices for recording adjustment which include theV-SEAT disclosed in the patent document 2, the L-SEAT indices accordingto this invention develop correlation with such conventional indices forevaluating the quality of reproduced signal as jitter and SbER, andtherefore can be regarded as excellent signal evaluation indices. Thus,according to this invention, the excellent signal evaluation indicesthat no conventional signal evaluation indices have ever paralleled inperformance, can be provided, and this fact is the third advantageattained by this invention. The empirical proof of this point will begiven later in reference to experimental results.

The affinity to the evaluation bit arrays used for evaluating thequality of reproduced signal in the calculation of SbER will now bedescribed. The techniques for evaluating reproduced signals disclosed inthe patent documents 1, 3 and 4 employ different constitutions butinclude in common the process of searching/extracting the first mostlikely evaluation bit array from among binary bit arrays outputed fromthe PRML decoder. The length M of the evaluation bit array can begeneralized to give the equation M=2N−1+2N_(2T), by using the constraintlength N in the PRML procedure and the number N_(2T) of continuous 2Tpatterns contained in the evaluation bit array. Here, N_(2T) denotes aninteger such as 0, 1, 2, . . . . According to the notation describedabove, N_(2T)=0, 1, 2 correspond to an edge shift, a 2T shift and a 2Tknock-on, respectively. Further, when N_(2T) is 0, 1, 2, 3, 4, 5 and 6,the corresponding Hamming distances are 1, 2, 3, 4, 5, 6 and 7,respectively, and the Hamming distance between the evaluation bit arraysof the patterns A and B is (N_(2T)+1). The evaluation bit array can beeasily enumerated from among 2^(M) bit arrays through the mechanicaloperations of extracting the relationship between the first most likelyevaluation bit array and the second evaluation bit array correspondingto the target signal that makes minimum the Euclidean distance from thetarget signal corresponding to the first evaluation bit array.

FIG. 15 is a table listing examples of evaluation bit arrays associatedwith PR (1,2,2,2,1) characteristic having a constraint length of 5.Similar bit arrays are disclose in the patent document 4. As is apparentfrom FIG. 15, in the case where evaluation bit arrays are searched andextracted from among binary bit arrays outputed from PRML decoder, usingPRML procedure having a constraint length of 5, the total of 108evaluation bit arrays, i.e. 54 pairs including 18 pairs for each of theHamming distances, are enumerated. In evaluating reproduced signals,these evaluation bit arrays must be searched and extractedsimultaneously.

FIG. 16 is a table listing the evaluation bit arrays associated with PR(1,2,2,2,1) characteristic having a constraint length of 5, shown inFIG. 15, in such an abridged way that those bit arrays which areequivalent to one another except their two head bits and two tail bits,are grouped together. As seen from FIG. 16, the 108 evaluation bitarrays associated with Hamming distances 1, 2 and 3 are represented bythe main bit arrays having bit lengths of 5, 7 and 9 and the auxiliarybit arrays XX and YY each having a bit length of 2 and attached to thefront and rear end of these main bit arrays. Here, the main bit arraysinclude four bit arrays such as “00011”, “00111”, “11100” and “11000”associated with Hamming distance of 1; four bit arrays such as“0001100”, “0011000”, “1110011” and “1100111” associated with Hammingdistance of 2; and four bit arrays such as “000110011”, “001100111”,“111001100” and “110011000” associated with Hamming distance of 3. Theauxiliary bit array AA is “00”, “10” or “11” and the auxiliary bit arrayBB is “00”, “01” or “11”. The intervals corresponding to the bit lengthsof the main bit arrays listed here are used as the intervals withinwhich the Euclidean distance between target signal and reproduced signalis calculated. The auxiliary bit arrays are only used to calculate thelevels of target signals at the front and rear ends of the main bitarrays and have nothing to do with the calculation of Euclideandistances among target signals. In this sense, the auxiliary bit arrayscan be considered to determine the boundary condition to determine thelevels of the target signals at their ends.

The main bit arrays can be determined independent of the constraintlength in PRML procedure. The reason for this will be described below.If the shortest run length m is set equal to 2T, then in order to showthat a single bit is changed due to an edge shift, the shortest lengthin terms of bit is obtained by multiplying m by 2 and adding 1 to 2m,that is, equal to 2m+1=5 bits. This is what the main bit looks like. Inlike manner, the generalization using the continuous 2T count N_(2T)included in each evaluation bit array yields the length of main bitarray equal to 2m+1+2N_(2T). Thus, the main bit array is meant as theshortest bit array determined depending on the continuous 2T countcontained in an evaluation bit array. On the other hand, as describedabove, the length of bit array needed to calculate the Euclideandistance from reproduced signal is represented, by using the constraintlength N in PRML procedure, as 2N−1+2N_(2T). The difference between thelengths of both the bit arrays is(2N−1+2N_(2T))−(2m+1+2N_(2T))=2(N−m−1), which is always an even number.This value is 2(N−3) if the shortest run length m=2.

As described above, if use is made of the main bit array independent ofthe constraint length N in PRML procedure and the auxiliary bit arrayshaving length (N−3) and attached at the front and rear ends of the mainbit array, then the evaluation bit array can be represented in someordered manner.

In this way, the ordered representation of each evaluation bit arrayenables the relationship between the index for evaluating the quality ofreproduced signal and this invention to be simplified and also the scaleof circuitry used in this invention to be reduced.

In the table of FIG. 16, evaluation bit arrays are classified into A andB groups in accordance with the disclosure of the patent document 4. Itis preferable from the viewpoint of reducing the scale of circuitry toselect the first evaluation bit array (i.e. evaluation bit arrayequivalent to target signal T) from among bit arrays obtained bybinarizing reproduced signals and then to generate the second evaluationbit array as secondary likelihood bit array (i.e. evaluation bit arrayequivalent to target signal L or R) on the basis of first evaluation bitarray. Since the Hamming distance between the first and secondevaluation bit arrays is previously determined, the second evaluationbit array can be generated by applying exclusive OR (XOR) operation tothe first evaluation bit array (T) and the generation bit array having1's whose number is equal to the Hamming distance. FIG. 17 is a tablewhich summarizes main bit arrays corresponding to Hamming distances 1˜7.In the column for the main bit arrays of the table, main bit arraysgiven above are enumerated. In this table, the numbers of the main bitarrays are determined by combining the Hamming distances and the numbers1˜4. As shown in the figure, the second main bit arrays can be obtainedthrough XOR operations using specific generation bit arrays peculiar toHamming distances. Also, the main bit array numbers of the second mainbit arrays are enumerated in the table.

As described above, the consideration of main bit arrays leads to theexplanation of the affinity between the evaluation bit array for SbERand the evaluation bit array according to the method of this invention.

FIG. 18 is a table listing main bit arrays for edge evaluation accordingto this invention in the case where the continuous 2T count is equal toor less than 2. When the L- and R-target generation bit arrays aresimultaneously generated to calculate the L-SEAT index, the length ofeach main bit array to be used is greater by 1T (i.e. one bit) than thatof the corresponding main bit array shown in the table of FIG. 17. Thus,the target generation arrays having Hamming distances of 1, 2 and 3 havebit lengths of 6, 8 and 10, respectively. In FIG. 18, just as in FIG.17, listed are the main bit arrays to be included in the bit arraysobtained through the binarization of reproduced signals and the L- andR-target generation bit arrays for generating the L- and R-targetsignals by performing XOR operations on themselves and the main bitarrays. The total number of the listed main bit arrays is 12, and theunderlined bit of each main bit array is the interested edge. The ruleadopted here is to select as the L- and R-target generation bit arraysthe main bit arrays whose interested edges are shifted by a single bitto the left and right, respectively, which satisfy the run lengthrestriction, and whose Hamming distances are minimized (i.e. the numberof inverted bits is minimized). Also, in the table in FIG. 18, therecorded mark is indicated by “1” and the space is indicated by “0”.Even when the amount of light reflected from the recorded mark is lessthan that of light reflected from the space, that is, in the case of theso-called High-to-Low type recording medium being used, the PR(1, 2, 2,2, 1) characteristic can be maintained if the “1” and “0” of the mainbit array are inverted so as to cause the recorded mark and the space tobe denoted by “0” and “1”, respectively. Alternatively, if all theimpulse responses are inverted with PR(−1, −2, −2, −2, −1)characteristic employed, the table shown in FIG. 18 can be used as itis. In the description of this invention given hereafter, the recordedmark is represented by “1” and the space by “0” unless otherwisespecified.

Description is made below of the relationship between the main bitarrays shown in FIG. 17 for calculating SbER and the main bit arraysshown in FIG. 18 for evaluating L-SEAT. FIGS. 19A and 19B illustrate thecomparison between both types of the main bit arrays in the case whereN_(2T)=0, that is, Hamming distance is 1. This comparison was made toperform the evaluation of the anterior edge of a recorded mark having alength of 3T or longer. The time instants at which the evaluation ismade are included with respect to SbER while the types of edges areincluded with respect to L-SEAT. As shown in these figures, each mainbit array includes only one edge. With respect to both SbER and L-SEAT,evaluation for two Hamming distances are made per edge. The main bitarrays for SbER coincides with the main bit arrays for L-Seat. Namely,the evaluation bit arrays for SbER and L-SEAT, including their auxiliarybit arrays, coincide with each other. In these figures, only theevaluation of the anterior edge is performed, but if “1” and “0” areinverted, the evaluation for the posterior edge can be likewiseperformed. It is evident in that case, too, that the evaluation bitarrays for SbER and L-SEAT remain coincident with each other.

FIG. 20 illustrates the comparison between both the main bit arrays inthe case where N_(2T)=1, that is, Hamming distance is 2. Each of themain bit arrays includes two edges. With respect to both SbER andL-SEAT, evaluation for two Hamming distances are made per edge. The mainbit array for SbER coincides with the main bit array for L-SEAT.Regarding the transition of evaluation with time, as indicated witharrows in the figure, it is apparent that evaluation is performed for L,L, R and R edges in this order named in case of SbER and for L, R, L andR edges in this order named in case of L-SEAT. The evaluation bit arraysfor SbER and L-SEAT would coincide with each other also in the case ofbit array patterns in which “1” and “0” are inverted.

FIG. 21 illustrates the comparison between the main bit arrays for bothSbER and L-SEAT in the case where N_(2T)=2, that is, Hamming distance is3. Each of the main bit arrays includes three edges. As in the aboveexample, with respect to both SbER and L-SEAT, evaluation for twoHamming distances are made per edge. The main bit arrays for SbERcoincide with the main bit arrays for L-Seat. Regarding the transitionof evaluation with time, as indicated with arrows in the figure, it isapparent that evaluation is performed for L, L, L, R, R and R edges inthis order named in case of SbER and for L, R, L, R, L and R edges inthis order named in case of L-SEAT. The evaluation bit arrays for SbERand L-SEAT would coincide with each other also in the case of bit arraypatterns in which “1” and “0” are inverted.

It has been revealed from the foregoing discussion that in the casewhere N_(2T) is equal to or less than 2, the evaluation bit arrays forcalculating SbER coincide with the main bit arrays for evaluation shownin FIG. 18. In like manner, with respect also to the case where N_(2T)is equal to or greater than 3, the evaluation bit arrays for SbER andL-SEAT can be likewise made coincident with each other if the main bitarrays for calculating L-SEAT, whose maximum Hamming distance is thesame as the maximum Hamming distance of the main bit arrays forcalculating SbER, satisfy the run length restriction and if the main bitarrays whose Hamming distances are minimum are selected as the main bitarrays for the L and R target signals. A concrete example in whichN_(2T) is equal to 3 will be described later. The basic concept ofL-SEAT is to evaluate the edge shift on the basis of the difference ofaverages of distributions of the target signals whose edges to beevaluated are shifted to the left and right, by focusing attention tosymmetricity, when the average of distributions of Euclidean distancedifferences differs from the difference of Hamming distances as shown inFIG. 5. According to this concept, the edge shift is evaluated on thebasis of the procedure (expressions D1˜D6) in which the extended edgeshift is evaluated at respective time instants, or the procedure(expressions 7˜13) in which the average of distributions ofindependently calculated Euclidean distance differences is evaluated.Further, not only main bit arrays for evaluation shown in FIG. 18 butalso various variations including a case of N_(2T)=3, can be used.

As described above, if the main bit arrays for evaluation shown in FIG.18 are evaluated on the basis of the equivalent edge shifts, the L-SEATcan be provided as an evaluation index with improved correlation withSbER or index of common concept, also from the viewpoint of the affinityof the evaluation main bit arrays. This point is the fourth improvementachieved by this invention.

[Requirement 3] Achievement of Record Adjustment in a Short Period ofTime

The evaluation index and the method of adjustment, capable ofindependent evaluation must be provided in accordance with the conditionfor recording pulses or the respective parameters of adaptive recordingpulses. Optical disc devices in general need to handle not only opticaldiscs of a particular standard but also optical discs for high-densityrecording such as CD, DVD, BD or other optical discs fabricated based onBDs. Adaptive recording pulses may vary in accordance with the standardsfor these different discs. It is also preferable that the proper indexfor evaluating such recording adjustments as, for example, the edgeshift along the time axis measured by a time interval analyzer, thejitter, the V-SEAT or the L-SEAT according to this invention, should beused depending on cases to be treated. In order to realize such aflexible index, a parameter table for recording adjustment has only tobe prepared first. Then, all-round measures can be provided byinstalling in the previous stage a circuit for calculating suchevaluation indices as the edge shifts of reproduced signals and the SNRfactors.

FIG. 22 shows in block diagram the configuration of such a circuit foradjusting recording condition. In FIG. 22, the read-out signal 51, whichhas been read out from an optical disc medium and passed through ananalog filter not shown in the figure, is converted to a digital signalhaving 6˜8 bits by an A/D converter 21, equalized by an automaticequalizer 22, and binarized by a PRML decoder 23 to be output as abinary bit array 52. A circuit 30 for evaluating the quality of thesignal for adjusting the write condition comprises edge evaluators 40,41 and 42; a selector 60; a write pulse evaluation table 35; and atiming adjuster 36. The edge evaluator 40 performs the evaluation of theedge shift along time axis with respect to each edge in a CD/DVD system;the edge evaluator 41 performs the evaluation of V-SEAT for BD; and theedge evaluator 42 performs the evaluation of L-SEAT for high-density BD.Each edge evaluator calculates the amount of edge shift, the extendededge shift or the SNR factor with respect to each edge. The selector 60selects the outputs of the edge evaluators depending on the kinds ofdiscs subjected to write/read operation. The write pulse evaluationtable 35 synchronizes the binarized bit array 52 with the edgeevaluation index outputted from the edge evaluator, performs theclassification of bit patterns in accordance with adaptive recordingpulses, allocates the bit patterns into, for example, a 4×4 table, andcalculates the average or standard deviation per table component. A CPU140 refers to the result obtained by the write pulse evaluation table 35and processes the adjustment of the respective parameters of theadaptive recording pulses. With this configuration described above, theparallel adjustments of parameters of the adaptive recording pulsesbecomes possible for different kinds of optical disc media. Accordingly,the adjustment of condition for write pulses can be achieved in a shortperiod of time and in a limited test write area, in comparison with themethod that uses a single evaluation index for read-out signal.

In this way, it has become possible to provide the evaluation index foradjustlong the recording condition, the adjusting method for recordingcondition and the optical disc device using the evaluation index and theadjusting method, which, regarding the adjustment of write conditioncorresponding to high-density recording with recording capacity of morethan 30 GB per disc surface based on BD systems, can solve the abovedescribed problems peculiar to the conventional techniques and which can(1) enjoy the high compatibility of the reproduction of data recorded inaccordance with the result of the adjustment, (2) guarantee that thequality of the data recorded in accordance with the result of theadjustment is sufficiently good when it is measured in terms of theevaluation index for reproduced signal such as SbER, and (3) adjust thecondition of the adaptive recording pulse in a short period of time. Thegist of this invention is the provision of the adjusting method forrecording condition and the optical disc device using this method,wherein when reproduced signals are evaluated by using the targetsignals having more than three Hamming distances (corresponding to=1, 2and 3) with respect to an optical disc using recorded marks whoseshortest run length is equal to 2T as in BD, the quality of theinterested edge is evaluated in accordance with the method forevaluating extended edge shifts at respective time instants or themethod for evaluating the average of distributions of independentlycalculated Euclidean distance differences; and the recording conditionis adjusted on the basis of the result of the evaluation.

As described above, an optical disc device capable of achievinghigh-density recording of 30 GB or higher in BD can be realized by usingthe adjusting method for recording condition which uses the L-SEATaccording to this invention as evaluating index.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in block diagram the configuration of the circuit forevaluating the reproduced signal, the circuit being used to realize anoptical disc device according to this invention;

FIG. 2 graphically shows the experimental results illustrating therelationship between recording power and bit error counts, measured byusing a 3-layer write-once optical disc fabricated for test purpose;

FIG. 3 graphically shows the results of simulations illustrating therelationship between SNR and SbER;

FIG. 4 graphically shows the experimenter result illustrating therelationship between bit error rate and SbER;

FIG. 5 shows examples of SAM distributions;

FIG. 6 shows the distributions for obtaining edge shifts Ec′ resultingfrom simulations;

FIG. 7 graphically shows the relationship between SNR and Ec′;

FIG. 8 graphically shows the relationship between reproduction power anddeviation of distribution center;

FIGS. 9A and 9B diagramatically show an example of equivalent edgeshift;

FIGS. 10A and 10B schematically show another example of equivalent edgeshift;

FIG. 11 shows the relationship between Hamming distance and cos θ;

FIGS. 12A˜12D show the correlations between dEDL and dEDR;

FIGS. 13A˜13D show the relationships between the averages of dEDR's anddEDR's and the extended edge shifts;

FIG. 14 graphically shows the relationship between SNR and Ec′, and theaverage Δ of extended edge shifts;

FIG. 15 is a table listing evaluation bit arrays associated withPR(1,2,2,2,1) characteristic;

FIG. 16 is a table listing evaluation bit arrays associated withPR(1,2,2,2,1) characteristic, the bit arrays being extracted accordingto similarities;

FIG. 17 is a table related to the main bit arrays and the generation ofthe second main bit arrays;

FIG. 18 is a table listing evaluation main bit arrays (N2T_(max)=2);

FIGS. 19A and 19B show the comparisons between evaluation main bitarrays for SbER and L-SEAT;

FIG. 20 shows comparisons between evaluation main bit arrays for SbERand L-SEAT;

FIG. 21 shows additional comparisons between evaluation main bit arraysfor SbER and L-SEAT;

FIG. 22 shows in block diagram an evaluation circuit;

FIG. 23 is a table listing evaluation main bit arrays (N2T_(max)=3);

FIG. 24 is a table as another embodiment, listing evaluation main bitarrays (N2T_(max)=2);

FIG. 25 is a table as another embodiment, listing evaluation main bitarrays (N2T_(max)=3);

FIGS. 26A˜26C show correspondences between evaluation bit arrays andwrite pulse tables;

FIGS. 27A˜27C show additional correspondences between evaluation bitarrays and write pulse tables;

FIGS. 28A and 28B illustrate the evaluations of edge shifts by usingL-SEAT;

FIGS. 29A and 29B illustrate another example of the evaluations of edgeshifts by using L-SEAT;

FIG. 30 shows L-SEAT distributions and SAM distributions;

FIGS. 31A and 31B graphically show the relationships between read powersand L-SEAT evaluation indices;

FIG. 32 shows in block diagram the structure of an automatic equalizerof symmetric type;

FIGS. 33A˜33C graphically show experimental results regarding recordingadjustment using L-SEAT;

FIGS. 34A˜34C graphically show additional experimental results regardingrecording adjustment using L-SEAT;

FIGS. 35A˜35C graphically show yet additional experimental resultsregarding recording adjustment using L-SEAT;

FIGS. 36A˜36C graphically show still additional experimental resultsregarding recording adjustment using L-SEAT;

FIG. 37 graphically shows power margin after write adjustment;

FIG. 38 shows the relationship between bit error rates and L-SEAT jitterand the relationship between bit error rates and V-SEAT jitter;

FIG. 39 diagrammatically shows how write pulses are adjusted;

FIG. 40 is the flow chart illustrating a method for write adjustmentaccording to this invention;

FIG. 41 graphically shows how focus is adjusted in accordance with theevaluation method according to this invention;

FIGS. 42A and 42B show the effects of adjustments achieved by the use ofextended write pulse and L-SEAT; and

FIG. 43 schematically shows the structure of an optical disc device.

DETAILED DESCRIPTION OF EMBODIMENTS

The adjusting method for recording condition and the optical disc deviceaccording to this invention will now be described by way of embodimentin reference to the attached drawings.

FIG. 23 is another embodiment of the table listing main bit arrays foredge evaluation according to this invention. The listed main bit arraysare for the case where N_(2T) is equal to 3. There are 20 main bitarrays listed in total. In each main bit array, the underlined bitcorresponds to an interested edge. The main bit arrays No. 1˜12 are thesame as those listed in the table shown in FIG. 18. The main bit arraysNo. 13˜20 correspond to the case where the continuous 2T count is 3. Asdescribed above, there is only one evaluation bit array that is thesecondary likelihood bit array, among main bit arrays listed in FIG. 15or FIG. 16. Accordingly, even in case of such a binarized bit array withN_(2T)=3 as “0000011001100”, the quality of reproduced signal isevaluated by regarding the bit array “0000110011000” having Hammingdistance=3 as the secondary likelihood evaluation bit array. On theother hand, this is not evaluated for the evaluation main bit arrayslisted in FIG. 15. In the case where such a bit array must beindependently evaluated depending on recording density and disc medium,that is, where when the continuous 2T counts are 2 and 3, the differencebetween the edge shifts of recorded 2T marks cannot be ignored, theevaluation main bit array shown in FIG. 23 must be used though the scaleof the circuit used in that case will be increased. Moreover, by usingevaluation main bit arrays shown in FIG. 23, the main bit array (No. 15)with the mark having a length of 3T or longer preceding Tsfp(2s, 2m) andthe main bit array (N. 17) with the mark having a length of 2T precedingTsfp(2s, 2m) can be separately evaluated. Further, in the case whereadaptive recording pulses are used for actual recording in accordancewith not only the length of a space preceding a record mark but also thelength of a mark appearing still further ahead, the evaluation main bitarrays shown in FIG. 23 can produce information on the recordingadjustment condition that exhibits one to one correspondence with therecording pulse table. Regarding the continuous 2T count (N_(2T))included in each evaluation main bit array, suitable one may be employedin consideration of such a circumstance as described above. The abovedescribed evaluation main bit array exhibits one to one correspondencewith the evaluation bit array for calculating SbER as in the case of themain bit array with N_(2T)=2. The description of the case where N_(2T)is 4 or greater, will be lengthy and therefore omitted here. However,those who are skilled in the optical disc technology will easily be ableto extend their idea to such a case from relationships of FIG. 18 andFIG. 23.

FIG. 24 is another embodiment of table listing main bit arrays for edgeevaluation according to this invention. Here is shown the case whereHamming distances for L and R target signals are set equal to each otherwhen N_(2T)=2. There are 12 main bit arrays listed in total. In eachmain bit array, the underlined bit corresponds to an interested edge.FIG. 24 differs from FIG. 18 in L- and R-target generation bit array andHamming distance. if the evaluation main bit arrays shown in FIG. 18 areused, their one-to-one correspondence with the bit arrays for evaluatingSbER cannot be maintained, but the deviation of distribution relative toSNR can be offset in principle. Accordingly, the dependence on SNR(constant in principle) better than the dependence on SNR shown in FIG.14 can be obtained. These evaluation main bit arrays may be employed ifthe change in SNR due to the change in drive device and recording mediumis regarded as the first priority item.

FIG. 25 is still another embodiment of table listing main bit arrays foredge evaluation according to this invention. Here is shown the casewhere Hamming distances for L and R target signals are set equal to eachother when N_(2T)=3. The feature of the table shown in FIG. 25 and theassociated evaluation performance are the same as those as describedwith respect to FIG. 24. In this case, too, a good dependence on SNR(constant in principle) can be obtained.

FIGS. 26A˜26C and FIGS. 27A˜27C show correspondences between evaluationmain bit arrays and recording pulse tables, as embodiments of thisinvention. FIGS. 26A˜26C shows the same main bit arrays as in FIG. 18and the recording pulse tables showing anterior edges Tsfp and posterioredges Telp with 4×4 different combinations of marks and spaces (referredto also as 4×4 type pulse table), giving associated main bit arraynumbers. As seen in FIGS. 26A˜26C, the main bit array of No. 9 may beused corresponding to the result of evaluation of Tsfp (2s, 2m). If acircuit for recording adjustment in a drive device is constructed inreference to this table, L-SEAT can be used to adjust the parameters ofthe recording pulses as defined in the 4×4 type pulse table. FIGS.27A˜27C show correspondences between evaluation main bit arrays and theassociated Hamming distances of the L- and R-target signals in such acase. In this way, the result of edge evaluation using L-SEAT can beeasily made coincident with the parameter table for the recordingpulses. The main bit arrays shown in FIGS. 23˜25 can be likewiseevaluated in accordance with the parameter table for the recordingpulses.

FIGS. 28A and 28B and FIGS. 29A and 29B show the simulation resultsillustrating examples of the edge evaluation using L-SEAT in accordancewith the above described parameter table for the recording pulses. Here,the condition for simulation is the same as that described above, therecording density is equivalent to 33 GB per disc surface as of a BD,and the PR class is (1, 2, 2, 2, 1).

The simulation in this case was performed when Tsfp(2s, 2m) is +0.2T(shift by 0.2T to the right). FIGS. 28A and 28B correspond to the casewhere the method for evaluating extended edge shifts at respective timeinstants (expressions (D1)˜(D6)) is used. It is apparent from FIG. 29Bthat when Tsfp(2s, 2m) is shifted to the right, the correspondingdistribution is also shifted to the right. A good recording conditioncan be obtained by adjusting the parameters of the recording pulse sothat respective edge shifts may approach zero. FIGS. 29A and 29Bcorrespond to the case where the method for evaluating the average ofdistributions of the independently calculated Euclidean distancedifferences (expressions (7)˜(13)) is used. In the simulation, theHamming distances in FIGS. 29A and 29B are both set equal to 2 for thelef and right shifts. As seen in FIG. 29A, when the shift of Tsfp(2s,2m) is zero, the averages of the L- and R-distributions differs from theideal Euclidean distance (=1), but both the averages are the same aseach other within a tolerable range of error. On the other hand, as seenin FIG. 29B, when the shift of Tsfp(2s, 2m) is not zero, the averages ofthe L- and R-distributions shift to the opposite directions. Therefore,if the parameters of the recording pulse are adjusted so that theaverages of the L- and R-distributions may coincide with each other, agood recording condition can be obtained. In this way, if the Hammingdistances of the main bit arrays for evaluating the L- and R-targetsignals are set equal to each other, the recording condition can beadjusted independent of SNR by using symmetricity. As described above,main bit arrays having different Hamming distances can also be employedas the main bit arrays for evaluating the L- and R-target signals.

FIG. 30 shows the comparison between the SAM distribution and the L-SEATdistribution, both distributions being obtained through simulation.Here, the condition for simulation is the same as that described above,the recording density is equivalent to 33 GB per disc surface as of aBD, and the PR class is (1, 2, 2, 2, 1).

It is ascertained that the average of SAM distribution approaches zeroas SNR decreases whereas the average of L-SEAT distribution remainsfixed to zero independent of SNR. Since the case where the evaluationmain bit arrays have N_(2T)=3 or greater, is an extended version of thecase shown in FIG. 30, the same result can be obtained.

FIGS. 31A and 31B graphically show the experimental results regardingthe dependence of L-SEAT indices on SNR. These results was obtained byperforming read experiments while the read power for the L0 layer of theabove mentioned 3-layer disc fabricated for test purpose, was beingchanged. The results correspond to the result shown in FIG. 8 obtainedaccording to the conventional technique. In FIGS. 31A and 31B, thehorizontal axis is graduated in read power with the read power of 1.2 mWnormalized as 100%. Since the amplitude of the read-out signal isproportional to the read power while the noise in the photodetector(amplifier noise) is constant, then in this experiment the SNR of theread-out signal is changed by changing read power. The L-SEAT jitter andL-SEAT shift were evaluated with the evaluation circuit shown in FIG.22, through allocation to the 4×4 type pulse table with respect to theanterior and posterior edges of a recorded mark. FIG. 31A shows themeasured values of L-SEAT jitter. The increase in the jitter with thedecrease in the read power reflects the change in SNR. On the otherhand, FIG. 31B shows the result of evaluation of edge shift with respectto Tsfp(2s, 2m). It is understood from FIG. 31B that the value of theL-SEAT shift remains constant independent of read power (SNR). Thischaracteristic is the feature of the method according to this inventionwhich makes it possible to evaluate the marginal evaluation based on thebasis of the Euclidean distance difference by separating the componentof the edge shift and the component depending on SNR from each other onthe basis of L-SEAT. It has therefore been ascertained that the use ofthis method enables the adjustment of write condition with high readcompatibility independent of the change in SNR caused depending on thedifference among drive devices and read/write conditions.

Now, description is made of an automatic equalizer suitable foradjusting the recording condition.

FIG. 32 shows in block diagram the structure of an automatic equalizerof symmetric type according to this invention. As described above, theuse of L-SEAT makes it possible to stabilize the adjustment of therecording pulses in response to change in SNR. On the other hand, drivedevices for actual use encounter (1) asymmetricity in the scanningdirection of light spot due mainly to the relative tilt (tangentialtilt) angle between disc medium and optical head, and (2) asymmetricityof read-out signal along the time axis due to the asymmetricity of thetap coefficients of the automatic equalizer. The distortion of read-outsignal along the time axis, which is detected as an edge shift, maybecome a disturbance in performing the adjustment of the write conditionwith high read compatibility. For example, even when a recorded mark hasany residual edge shift, if the automatic equalizer makes its tapcoefficients asymmetric so that the residual edge shift can becompensated, the recorded mark will be judged such that the measurededge shift is small and hence that the recording was quite successful.In general, different drive manufacturers produce many different sortsof optical disc drives and such different optical disc drives use somany different circuit configurations. Accordingly, the recording ofdata in such a manner that only a particular drive can easily reproducethe recorded data will create a problem that must be solved in opticaldisc systems in which compatibility of recording media is highlyrequired. The automatic equalizer of symmetric type shown in FIG. 32 canprovide a solution to this problem. In FIG. 32, a read-out signal 51reproduced from an optical disc medium (not shown) is converted todigital data by means of an A/D converter (not shown); the digital dataare equalized by the automatic equalizer 22; and the output of theautomatic equalizer 22 is then binarized by a PRML decoder 23 so that abinary bit array 52 is outputted. The tap coefficients C₀, C₁, C₂ . . .are subjected to an automatic learning process so that the RMS errorbetween the target signal corresponding to the binary bit array 52 andthe signal outputed from the automatic equalizer 22 can be minimized.This algorithm is usually called “LMS (Least Mean Square) Method” andperformed by a LMS circuit 62. The renewed tap coefficients a₀, a₁, a₂,. . . , created by the LMS circuit 62, are temporarily stored in abuffer 64. In a work register 65 used for the actual operation of a FIRfilter are set the averages each of which is the average of tapcoefficients located symmetrically with each other along the time axis(e.g. average of a₀ and a_(n), a₁ and a_(n-1), etc.). In this way, thetap coefficients of the automatic equalizer are symmetricized so thatthe reproduction of recorded marks with distorted edge shifts can beprevented. Incidentally, the I-V converting amplifier included in aphotodetector and other fulters may sometimes generate group delay dueto circuit configuration. The provision, if necessary, of a group delaycompensator 61 can reduce such group delay. The group delay compensator61 can be embodied by the use of a FIR filter having asymmetric tapcoefficients each of which is a preset value. Further, with this circuitconfiguration, it becomes possible to reduce the asymmetricity of lightspot in the direction of time axis by adjusting the amount of tangentialtilt in such a manner that SbER or L-SEAT jitter is minimized while thedata in a well-recorded reference disc is being reproduced. With thiscircuit configuration, it can be made possible for the automaticequalizer to act solely on the adjustment of the frequencycharacteristic of reproduced signal. The automatic equalizer ofsymmetric type according to this invention can provide a recordingcondition of high reproduction compatability not only when it iscombined with the L-SEAT but also even when it is combined with any ofconventional record adjustment methods. Since the output of the LMScircuit 62 can be transferred directly to the buffer 64 through theaddition of a suitable circuit such as a selector, the automaticequalizer of symmetric type according to this invention can be easilyoperated as an ordinarily automatic equalizer (having no tapsymmetricity restriction).

The description to follow is of the result obtained by the use of anautomatic equalizer of symmetric type having 21 taps.

FIG. 33A˜33C, FIGS. 34A˜34C, FIGS. 35A˜35C and FIGS. 36A˜36C graphicallyshow the experimental results regarding the adjustments of write pulseconditions by using L-SEAT indices. In these experiments, the L-SEATjitters, L-SEAT shifts and SbERs were measured while changing the fourwrite pulse parameters such as Tsfp(2s, 2m), Tsfp(3s, 2m), Tsfp(2s, 3m)and Tsfp(3s, 3m) in the L0 layer of the 3-layer test disc mentionedabove. SbERs were measured without maintaining the symmetricityrestriction on the tap coefficients of the automatic equalizer just asin the ordinary reproduction procedure. The basic unit used in adjustingthe edge of write pulse was set equal to T/64, and the linear speed forwriting and reading was set equal to double the speed of data transferin case of BDs. As seen from these figures, the pulse edge positionsthat give the zero of L-SEAT shift and the valley bottoms of L-SEATjitter and SbER, coincide with one another within a tolerance smallerthan T/64. Since the adjustment unit for pulse width is usually setequal to T/16, it was ascertained from these results that the adjustmentof recording condition can be performed very well by using the L-SEATshifts and the L-SEAT jitters. As a result of having performed suchadjustments on all the four parameters of write pulses, the value ofSbER has been improved from 3×10⁻³ to 1×10⁻⁷. FIG. 37 graphically showsthe relationship obtained by measurement between write power and biterror rate. A sufficient power margin of about ±10% came to be obtained.

FIG. 38 graphically shows the relationships obtained through experimentsbetween bit error rate and L-SEAT jitter and between bit error rate andV-SEAT jitter. In these experiments, the relationships between bit errorrate and L-SEAT jitter and between bit error rate and V-SEAT jitter weremeasured while varying write power, defocusing, spherical aberration andtangential and radial tilts of disc media. It was ascertained from FIG.38 that the correlation between bid error rate and jitter was furtherimprove for L-SEAT than for V-SEAT. The reason for this is as describedabove.

The adjustment method for recording condition according to thisinvention will now be described on the basis of the results of theexperiments and simulations described above.

FIG. 39 illustrates an example of how the adaptive parameters of writepulse are adjusted. In FIG. 39, the adaptive parameters of write pulseare explained by the help of 4×4 type pulse tables. The results ofmeasurement of L-SEAT shifts and jitters are allocated to the 4×4 typepulse tables as described above. At this time, data are written in anoptical disc medium while varying the write pulse condition, the valueof the corresponding L-SEAT shift is evaluated by reading out thewritten data, and the parameters of the write pulse are so determined asto minimize the shift value. In this way, a good condition for the writepulse can be obtained. As seen from the results shown in FIGS. 33A-33Cthrough FIGS. 36A-36C, adjustment results more stable against variouschanges come to be obtained if adjustment is performed to reach not onlythe condition for the minimized L-SEAT shift but also the condition forthe minimized L-SEAT jitter. As apparent from this example, since thewrite pulse parameters exhibit a one-to-one correspondence with theevaluation values therefor, the simultaneous adjustment of plural writepulse parameters can be parallelly performed if write/read operation iscarried out while changing the plural write pulse parameters at the sametime. By doing so, it becomes possible to shorten the time for testwriting in a drive device to a great extent. To be concrete, although adouble-speed drive device using the method for determining the writepulse parameters one by one, takes process time of about 30 seconds toone minute, the parallel process using this method will be able tocomplete such test writing in about one second. In applying thisadjusting method, if there are any fixed parameter among the entirewrite pulse parameters, adjustment can be stabilized. In general, it ispreferable to fix such parameters associated with the formation of longmarks as, for example, Tsfp(5s, 5m) and Telp(5s, 5m).

FIG. 40 is the flow chart illustrating the entire procedure of adjustingwrite pulse condition. To begin with, in Step 101, the group delay inthe reproduction circuit shown in FIG. 32 is checked, if necessary, todetermine the condition for compensation of the group delay. Then, inStep 102, the operating mode of the automatic equalizer is set to thesymmetric mode. In Step 103, while reference data are being read out,defocusing amount, spherical aberration and the tilt of disc medium areadjusted in such a manner that such indices for read-out evaluation asSbER and L-SEAT jitter are optimized. As described above, the tangentialtilt must be adjusted with special consideration, such as by reading outplural reference data or by including the condition for optimizing thewrite sensitivity. In Step 104, while the symmetrisuty, S/N ratio andcrosstalk of the read-out signal are being taken into consideration,proper conditions for the basic pulse and power are determined by usingwritten data including marks and spaces having a length of 5T orgreater. By doing so, the write pulse parameters for long markscorresponding to Tsfp(5s, 5m) and Telp(5s, 5m) shown in the 4×4 typepulse table is fixed. Tsfp(5s, 5m) corresponds to the write pulsecondition for the anterior edge, and Telp(5s, 5m) to the write pulsecondition for the posterior edge. In Steps 105 and 106, while adaptivewrite pulse parameters are being adjusted, adjustment is continued untilthe residual edges shaft becomes less than a preset value (e.g. ±0.1% ofT). In Step 107, the performance of write pulse is evaluated byevaluating the valley bottom values for SbER and bit error rate and thepower margin with respect to the obtained write pulse, and decision ismade on whether a predetermined performance has been achieved. If theresult of the decision indicates that the achieved performance isinsufficient, the flow returns to Step 104, where like adjustment isperformed while the base pulse and the power level are being changed.When the predetermined performance has been achieved as a result of thisseries of steps, adjustment is finished.

FIG. 41 graphically shows the relationship obtained through experimentbetween focus offset and SbER. In this experiment, the automaticequalizer of symmetric type according to this invention was used. Thefocus offset can be adjusted to a proper value by using thisrelationship and making SbER minimum. The same procedure can also beused for the adjustment of redial tilt, tangential tilt, sphericalaberration, etc. The Step 103 in FIG. 40 can be carried out according tothis procedure.

Description is now made of write pulses adapted to high-densityrecording. When the high-density recording with recording capacity of 30GB or higher is to be performed on the basis of the BD standard, theadjacent marks are largely affected by thermal interference between themsince the length of a 2T mark or space is about 100 nm, which is smallerthan the size of the used light spot (about 500 nm, wavelength 405 nm,NA 0.85). Such an adverse effect becomes remarkable especially in thecase of a multiple-layer discs in which it is impossible to make themetal reflection film serving as thermal buffer thick enough from theviewpoint of achieving satisfactory transparency. In such a case, it isconsidered difficult to form satisfactory recorded marks even withadaptive write pulses that can be determined depending only on thelength of a recorded mark and the lengths of the spaces immediatelyanterior and posterior to the recorded mark. In such a case, the patternwhich is affected by the thermal interference to the greatest extent isthe one consisting of continuous 2T mark and 2T space. As describedabove, this pattern is that which brings about the highest errorfrequency. Therefore, in the case where such continuous 2T mark and 2Tspace occur, it is useful to regard such a pattern as a precedingpattern and to extend the adaptive write pulse table.

FIGS. 42A and 42B illustrate the effect obtained through the adjustmentby the use of extended adaptive write pulse table and L-SEAT. Theextended table shown here was obtained by the addition of an adaptivetable under the assumption that the write pulses used are the standardwrite pulses for BD and that the pattern of continuous 2T mark and 2Tspace is regarded as a preceding space. As described above, if the tableshowing in FIG. 23 is used as the table of evaluation main bit arrays,it is possible to evaluate edge shifts in accordance with write pulsesby the use of L-SEAT. As seen from FIG. 42B, the amount of the residualshift for Telp(2s, 2m) was able to be improved to a greater extent, ascompared with the case where the standard write pulses for BD were used.Here, the adjustment unit for the write pulse width was set equal toT/32.

Now, description is made of an optical disc device as an embodiment ofthis invention.

FIG. 1 shows in block diagram the configuration of a circuit forevaluating read-out signals, the circuit being designed for realizing anoptical disc device as an embodiment of this invention. In FIG. 1, theread-out signal 51, which has been read out of an optical disc mediumand passed through an analog filter not shown in the figure, isconverted to a digital signal having 6˜8 bits by an A/D converter 21,equalized by an automatic equalizer 22, and binarized by a PRML decoder23 to output a binary bit array 52. A circuit 30 for evaluating thequality of the read-out signal that calculates L-SEAT index, comprisesmain bit array detector 31, left & right shift bit array generator 32,ED (Euclidean Distance) difference calculator 33, write controlparameter table sorter 34, and summary data table 35. The main bit arraydetector 31 stores data corresponding to the preselected main bit arraysand judges whether a preselected main bit array is included in thebinary signal 52. When the binary signal 52 includes a preselected mainbit array, the left & right shift bit array generator 32 performs theXOR process as described with, for example, FIG. 18 and generatesevaluation main bit arrays for L- and R-target signals. The ED(Euclidean Distance) difference calculator 33 calculates the Euclideandistances among the evaluation main bit arrays for the T-, L- andR-target signal and the equalized write signals 53 outputted from theautomatic equalizer 22. The write control parameter table sorter 34statistically processes the calculated Euclidean distances in a wayaccording to the adaptive write pulse table, in accordance with themethod (expressions (D1) through (D6)) for evaluating the extended edgeshifts at respective time instants or the method (expressions (7)through (13)) for evaluating the average of distributions ofindependently calculated Euclidean distance differences. The summarydata table 35 obtains such tables as shown in, for example, FIG. 39. TheCPU 140 refers to the summary data table 35, changes the parameters ofthe write pulse through controlling a write pulse adjustment circuit notshown in the figure, and adjusts the parameters of the write pulseaccording to the method shown in FIG. 40.

FIG. 43 schematically shows the structure of an optical disc deviceincorporating the method for evaluating the read-out signals accordingto this invention. An optical disc medium 100 mounted on the device isrotated by means of a spindle motor 160. At the time of reading, alaser-power/pulse controller 120 controls the current flowing throughthe semiconductor laser 112 via the laser driver 116 in an optical head110 so as to generate laser light 114 whose intensity is adjusted to thelevel instructed by the CPU 140. The laser light 114 is focused by anobjective lens 111, to form a light spot 101 on the optical disc medium100. The light beam reflected from the light spot 101 is passed throughthe objective lens 111 to be focussed on and detected by, aphotodetector 113. The photodetector 113 comprises a plurality of splitphoto-detecting elements. A read-out signal pre-processor 130 reproducesthe information recorded in the optical disc medium 100 on the basis ofthe signal detected by the optical head 110. The read-out signalpre-processor 130 incorporates therein this invention shown as a circuitblock in FIG. 1. With this configuration, the optical disc deviceaccording to this invention can work as a device for realizing a BDsystem having recording density of 30 GB per disc, optimize the writepulse condition through test writing, and secure a good system marginand read compatibility.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An adjusting method for recording condition for use with an opticaldisc device that records data into an optical disc medium by using codeswhose shortest run length is 2T and reproduces the recorded data byusing the adaptive equalizing procedure and the PRML procedure,comprising a step of using a means for providing a limitation on theadaptive equalizing procedure in such a manner that the tap coefficientsare symmetric with respect to the center along the time axis; a step ofgenerating a first binary bit array by binarizing the reproduced signalwaveform reproduced from the optical disc medium in accordance with thePRML procedure; a step of generating a second binary bit array and athird binary bit array whose shortest run length is equal to or greaterthan 2T, and which are obtained by shifting the interested edge of thefirst binary bit array to the left and right, respectively; a step ofgenerating a first, a second and a third target signal waveformscorresponding to the first, second and third binary bit arrays,respectively; a step of generating a first value equivalent to thedifference between the Euclidean distance between the second targetsignal waveform and the reproduced signal waveform and the Euclideandistance between the first target signal waveform and the reproducedsignal waveform, and a second value equivalent to the difference betweenthe Euclidean distance between the third target signal waveform and thereproduced signal waveform and the Euclidean distance between the firsttarget signal waveform and the reproduced signal waveform; a step ofcalculating the value for evaluating the shift of the interested edge byusing the difference between the first and second values; and a step ofadjusting the recording condition by using the value for evaluating theshift of the interested edge.
 2. An adjusting method for recordingcondition as claimed in claim 1, wherein the first value is used afterit has been normalized by the Euclidean distance between the firsttarget signal waveform and the second target signal waveform, and thesecond value is used after it has been normalized by the Euclideandistance between the first target signal waveform and the third targetsignal waveform, and wherein as the shift evaluation value is used theaverage of first evaluation values, each based on the difference betweenthe first and second values; the sum of the squares of the standarddeviation of the first evaluation values and the standard deviation ofthe second evaluation values, each based on the sum of the first andsecond values; or the difference between the average of the first valuesand the average of the second values.
 3. An optical disc device havingthe function of recording data into an optical disc medium by usingcodes whose shortest run length is 2T and reproducing the recorded databy using the adaptive equalizing procedure and the PRML procedure,comprising a means for providing a limitation on the adaptive equalizingprocedure in such a manner that the tap coefficients are symmetric withrespect to the center along the time axis; a means for generating afirst binary bit array by binarizing the reproduced signal waveformreproduced from the optical disc medium in accordance with the PRMLprocedure; a means for generating a second binary bit array and a thirdbinary bit array whose shortest run length is equal to or greater than2T, and which are obtained by shifting the interested edge of the firstbinary bit array to the left and right, respectively; a means forgenerating a first, a second and a third target signal waveformscorresponding to the first, second and third binary bit arrays,respectively; a means for generating a first value equivalent to thedifference between the Euclidean distance between the second targetsignal waveform and the reproduced signal waveform and the Euclideandistance between the first target signal waveform and the reproducedsignal waveform, and a second value equivalent to the difference betweenthe Euclidean distance between the third target signal waveform and thereproduced signal waveform and the Euclidean distance between the firsttarget signal waveform and the reproduced signal waveform; and a meansfor adjusting the recording condition for the optical disc medium byusing the difference between the first and second values.
 4. Anadjusting method for recording condition for use with an optical discdevice that records data into an optical disc medium by using codeswhose shortest run length is 2T and reproduces the recorded data byusing the adaptive equalizing procedure and the PRML procedure, whereinthe recording condition is so adjusted that the reproduced signal withdesired quality can be obtained, by using as the adaptive equalizingprocedure one that provides such a limitation that the tap coefficientsare symmetric with respect to the center along the time axis.
 5. Anadjusting method for recording condition as claimed in claim 1, whereinthe recording condition is adjusted after at least one of the radialtilt of disc, the tangential tilt of disc, the aberration of focus, thespherical aberration due to improper adjustment of the optical head beamexpander has been previously determined.
 6. A data reproduction methodfor reproducing data recorded using codes whose shortest run length is2T, by using the adaptive equalizing procedure and the PRML procedure,wherein at least one of the radial tilt of disc, the tangential tilt ofdisc, the aberration of focus, the spherical aberration due to improperadjustment of the optical head beam expander is so determined that thereproduced signal with desired quality can be obtained, by using as theadaptive equalizing procedure one that provides such a limitation thatthe tap coefficients are symmetric with respect to the center along thetime axis.
 7. An adjusting method for recording condition for use withan optical disc device that records data into an optical disc medium byusing codes whose shortest run length is 2T and reproduces the recordeddata by using the adaptive equalizing procedure and the PRML procedure,wherein a step of generating a first binary bit array by binarizing thereproduced signal waveform reproduced from the optical disc medium inaccordance with the PRML procedure; a step of generating a second binarybit array and a third binary bit array whose shortest run length isequal to or greater than 2T, and which are obtained by shifting theinterested edge of the first binary bit array to the left and right,respectively; a step of generating a first, a second and a third targetsignal waveforms corresponding to the first, second and third binary bitarrays, respectively; a step of generating a first value equivalent tothe difference between the Euclidean distance between the second targetsignal waveform and the reproduced signal waveform and the Euclideandistance between the first target signal waveform and the reproducedsignal waveform, and a second value equivalent to the difference betweenthe Euclidean distance between the third target signal waveform and thereproduced signal waveform and the Euclidean distance between the firsttarget signal waveform and the reproduced signal waveform; and a step ofcalculating the value for evaluating the shift of the interested edge byusing the difference between the first and second values, wherein therecording condition is adjusted by using the value for evaluating theshift of the interested edge.
 8. A method for recording data comprisinga step of using as adaptive equalizing procedure that provides such alimitation that the tap coefficients are symmetric with respect to thecenter along the time axis; a step of generating a first binary bitarray by binarizing the reproduced signal waveform reproduced from theoptical disc medium in accordance with the PRML procedure; a step ofgenerating a second binary bit array and a third binary bit array whoseshortest run length is equal to or greater than 2T, and which areobtained by shifting the interested edge of the first binary bit arrayto the left and right, respectively; a step of generating a first, asecond and a third target signal waveforms corresponding to the first,second and third binary bit arrays, respectively; a step of generating afirst value equivalent to the difference between the Euclidean distancebetween the second target signal waveform and the reproduced signalwaveform and the Euclidean distance between the first target signalwaveform and the reproduced signal waveform, and a second valueequivalent to the difference between the Euclidean distance between thethird target signal waveform and the reproduced signal waveform and theEuclidean distance between the first target signal waveform and thereproduced signal waveform; a step of calculating the value forevaluating the shift of the interested edge by using the differencebetween the first and second values; and a step of adjusting therecording condition by using the value for evaluating the shift of theinterested edge, wherein data is recorded using codes whose shortest runlength is 2T, on the basis of the adjusted recording condition.
 9. Anadjusting method for recording condition as claimed in claim 1, whereinthe Hamming distance between the first binary bit array and the secondbinary bit array is equal to the Hamming distance between the firstbinary bit array and the third binary bit array.